When it comes to testing your website's performance and quality, three names come up constantly: WebsiteLinter, Google Lighthouse, and GTmetrix. They're all free (or have free tiers). They all measure something about your site's health. But they're built for different things, and choosing the wrong one means you'll waste time debugging the wrong problems.
This guide breaks down each tool honestly — what it does well, where it falls short, and when to use it.
The 30-Second Summary
- WebsiteLinter — Best when you need a complete site health picture: SEO + performance + security in one report
- Lighthouse — Best for performance deep-dives and understanding exactly what to fix and why
- GTmetrix — Best for waterfall analysis and diagnosing slow third-party requests
→ Run a free WebsiteLinter scan here (no account required) to get started, then dive into this guide to understand which tool works best for your specific needs.
Now let's go deeper.
WebsiteLinter
WebsiteLinter is an all-in-one scanning tool designed for web professionals, agency owners, and business owners who want a complete audit without running three separate tools.
What It Checks
WebsiteLinter pulls from multiple data sources in a single scan:
Performance:
- Google PageSpeed Insights scores (mobile and desktop)
- Core Web Vitals: LCP, INP, CLS
- Time to First Byte (TTFB)
- Total page weight and resource count
SEO:
- Title tag and meta description (length, presence, uniqueness flags)
- Canonical URL
- robots.txt detection and validation
- XML sitemap detection
- Open Graph and Twitter Card tags
Security (its standout feature):
- HTTP security headers: CSP, HSTS, X-Frame-Options, X-Content-Type-Options, Referrer-Policy, Permissions-Policy
- SSL certificate validity and expiry
- Weighted security score (0–100) with a letter grade
Accessibility:
- Image alt text coverage
- Basic ARIA landmark detection
- Color contrast issues
Where It Excels
The security scoring is genuinely unique among free tools. Most performance tools don't touch HTTP headers at all — WebsiteLinter makes them a first-class metric with a weighted grade that accounts for header criticality.
The all-in-one format is also valuable when you need to quickly evaluate a client's site or compare your site against a competitor. Instead of toggling between tabs and reconciling different scores, you get one report with consistent data.
Where It Falls Short
WebsiteLinter gives you the what clearly, but for some performance issues the why requires Lighthouse's diagnostic depth. It doesn't show a waterfall chart, and it scans a single URL at a time in the free tier.
Best used by: Freelancers, agencies, and business owners doing site audits or competitive analysis.
Google Lighthouse
Lighthouse is Google's open-source auditing tool. It's available in Chrome DevTools, as a CLI tool, and through PageSpeed Insights. Every time you run PageSpeed Insights, you're running Lighthouse under the hood.
What It Checks
Lighthouse organizes its findings into five categories:
Performance:
The most detailed performance reporting available. It breaks down every metric with specific recommendations and estimates the savings from each fix. Key diagnostics include:
- Render-blocking resources
- Unused JavaScript and CSS
- Image sizing and format optimization
- Third-party script impact
- Server response time
Accessibility:
Automated accessibility checks against WCAG guidelines — missing form labels, low contrast ratios, missing ARIA attributes, keyboard navigation issues.
Best Practices:
Security-adjacent checks: HTTPS enforcement, safe browser APIs, deprecated APIs, console errors.
SEO:
Basic on-page SEO: title tag presence, meta description, mobile-friendly text size, crawlable links.
PWA (Progressive Web App):
Checks for service workers, web app manifests, and offline capability — mostly relevant for app-like experiences.
Where It Excels
Lighthouse's performance diagnostics are the industry standard. When PageSpeed reports that your LCP is 4.3 seconds, Lighthouse shows you exactly which element is the LCP target, why it's slow, and what changes would fix it. The "Opportunities" section provides concrete, estimated savings (e.g., "Reduce unused JavaScript: estimated savings of 1.2s").
It's also the most trustworthy source for your PageSpeed score because it is PageSpeed — running it locally in Chrome DevTools gives you controlled lab conditions without network variability.
Where It Falls Short
Lighthouse runs in lab conditions, which means simulated throttling on a controlled machine. Real-user performance (field data) can differ significantly. A site can score 90+ in Lighthouse while having poor CWV field data from Chrome users.
It also doesn't check security headers, backlinks, or give you a site-wide view. It's a per-page, performance-focused tool.
Best used by: Developers optimizing specific pages, anyone debugging Core Web Vitals, teams running automated performance CI checks.
GTmetrix
GTmetrix is a web performance testing service that combines Lighthouse scores with a detailed waterfall chart and additional analysis. It's been around since 2009 and has a loyal following among developers and performance engineers.
What It Checks
Lighthouse scores:
GTmetrix runs the same Lighthouse checks as PageSpeed Insights, so you get the same performance, accessibility, best practices, and SEO scores.
Waterfall chart:
The waterfall is GTmetrix's signature feature. It shows every single network request the page makes — HTML, CSS, JavaScript, fonts, images, third-party scripts — in timeline form. You can see:
- Which requests block the critical rendering path
- How long each resource takes to load
- Which third-party domains are adding latency
- Whether HTTP/2 multiplexing is being used
Video playback:
GTmetrix records a video of the page loading and creates a filmstrip view showing exactly what the user sees at each second. This is invaluable for diagnosing visual stability issues (CLS) and perceived load performance.
Historical tracking:
The free tier lets you save and compare multiple test runs over time, which is useful for confirming that your optimizations actually helped.
Where It Excels
The waterfall chart is unmatched for diagnosing slow third-party requests. If a slow analytics script, chat widget, or ad network is hurting your load time, the waterfall will show you exactly which domain it's coming from, how long it's blocking rendering, and how it compares to your own resources.
GTmetrix also lets you test from multiple global locations (limited on free tier), which is useful for sites serving international audiences.
Where It Falls Short
GTmetrix free tier restricts you to a small number of tests per day, and mobile testing locations require a paid plan. Like Lighthouse, it's performance-only — no SEO signals, no security header checks.
The interface can feel overwhelming for non-technical users. The waterfall chart is powerful, but it takes experience to interpret correctly.
Best used by: Performance engineers, developers chasing down specific slow requests, anyone debugging third-party script impact.
Side-by-Side Comparison
| Feature | WebsiteLinter | Lighthouse | GTmetrix |
|---|---|---|---|
| Core Web Vitals | ✅ | ✅ Lab data | ✅ Lab data |
| Performance Score | ✅ | ✅ | ✅ |
| Security Headers | ✅ Full grade | Partial | ❌ |
| SEO Signals | ✅ Full | Basic | Basic |
| Accessibility | ✅ | ✅ Full | ✅ |
| Waterfall Chart | ❌ | ❌ | ✅ |
| Video Playback | ❌ | ❌ | ✅ |
| Historical Tracking | ❌ | ❌ | ✅ (limited) |
| Multi-location Testing | ❌ | ❌ | ✅ (limited) |
| No Account Required | ✅ | ✅ | ✅ |
| Free Tier | ✅ Unlimited | ✅ | Limited |
| Continuous Monitoring | ✅ Paid tier — recurring audits across SEO + Performance + Security + Accessibility | ❌ Point-in-time only | ❌ History tracking only (no alerts) |
WebsiteLinter is the only tool in this comparison that offers recurring multi-category health audits. Lighthouse and GTmetrix are point-in-time tools by design.
Which Tool for Which Job
Running a site audit for a client:
Start with WebsiteLinter. The all-in-one report covers everything you need to present findings, including the security grade which clients don't expect and find impressive.
Optimizing a specific page for PageSpeed:
Use Lighthouse in Chrome DevTools or PageSpeed Insights. The per-element diagnostics and estimated savings guide your optimization work better than any other tool.
Debugging a slow page load with third-party scripts:
Use GTmetrix for the waterfall. Identify which external domains are adding latency and in what order they're loading.
Comparing your site against a competitor:
Use WebsiteLinter to scan both URLs and compare scores across performance, security, and SEO simultaneously. → See our guide to Semrush site audit alternatives if you're evaluating multiple competitive tools for your workflow.
Setting up automated performance monitoring in CI/CD:
Use Lighthouse CLI or a Lighthouse-based tool like lhci (Lighthouse CI). It's the only one of these three that runs as a proper CLI tool.
Setting up continuous website monitoring for your live site:
Use WebsiteLinter's paid tier. Of the three tools compared here, only WebsiteLinter offers recurring scheduled audits — covering SEO, performance, accessibility, and security in a single recurring report. Lighthouse and GTmetrix are built for manual, on-demand diagnostics; they have no alerting or recurring scan capability for live sites.
Continuous Website Monitoring: Why One-Time Scans Miss Issues
Lighthouse and GTmetrix are exceptional diagnostic tools — but they're point-in-time tools. You run them when you decide to run them. Issues that develop between manual checks go undetected.
Consider what can change on a live website between audits:
- A plugin update breaks a security header configuration
- A new image goes live without compression, hurting Core Web Vitals
- A third-party script slows Time to First Byte
- A form loses its ARIA label after a theme update
WebsiteLinter's paid tier addresses this by running recurring website health audits on a schedule you configure — giving you a consistent, up-to-date view of your site's SEO health, Core Web Vitals, security grade, and accessibility compliance without requiring manual intervention.
This is the key differentiator in this comparison: Lighthouse and GTmetrix are tools you use reactively (when performance drops or before a launch). WebsiteLinter's paid tier functions proactively — alerting you to changes before users or Google notice them.
→ Comparing monitoring solutions? Check out our Ahrefs vs WebsiteLinter comparison for another look at how modern audit tools handle continuous monitoring.
🏆 Best for SMB Continuous Monitoring
WebsiteLinter's paid tier offers recurring website health audits covering SEO + Performance + Accessibility + Security — the only tool in this comparison purpose-built for ongoing site monitoring, not just one-off diagnostics. Lighthouse and GTmetrix remain superior for deep performance debugging, but neither offers recurring multi-category monitoring. See WebsiteLinter pricing →
The Honest Answer
These tools aren't rivals — they're complementary. The best workflow for most sites:
- Start with WebsiteLinter for a complete health baseline
- Use Lighthouse when you need to diagnose a specific performance issue
- Use GTmetrix when you need to understand request-level behavior
All three are free to get started. If you haven't scanned your site yet, begin where you'll get the most information the fastest.
Run a free WebsiteLinter scan at websitelinter.com — performance, security, and SEO in one report, no account required. Need recurring monitoring? See WebsiteLinter pricing →