Find what's slowing down your website.
Audit your code and resources, uncover potential bottlenecks, and get a prioritized optimization plan.
Analyze your resources
Paste source code or upload local files. Your content stays in your browser.
◆ Analysis runs locally. Files are not uploaded to FastTL.
Performance audit
Issue severity
Prioritize the highest-impact findings first.
Detected bottlenecks
Evidence and recommended next steps for each finding.
Resource inventory
Detected file types, sizes, and relevant code patterns.
| Resource | Type | Size | Notes |
|---|
Optimization checklist
Track the actions you have completed.
This is a static code and resource audit, not a live browser speed test. Findings are heuristic; verify changes with a browser performance profiler and real-user metrics.
FREE WEB DEVELOPMENT TOOL
Find What’s Slowing Down Your Website
Identify potential website performance bottlenecks in your HTML, CSS, and JavaScript. Review browser timing data, inspect resource-loading patterns, and discover practical opportunities to improve your website’s performance—all with a browser-based workflow.
What Is the Website Performance Bottleneck Analyzer?
The Website Performance Bottleneck Analyzer is a free tool designed to help developers, website owners, SEO professionals, and beginners investigate common frontend performance issues.
A website may feel slow because of large source files, render-blocking scripts, inefficient stylesheet patterns, oversized images, or resources that take too long to load. This tool helps you inspect source-code patterns and review browser performance information so you can decide what to investigate first.
Instead of relying on a single speed score, you can combine source inspection with timing data collected by your browser to better understand possible performance problems.
What Can You Analyze?
HTML Source Analysis
Inspect HTML source text for common patterns that may affect page structure, loading behavior, and frontend efficiency.
CSS Performance Checks
Review stylesheet patterns, repeated stylesheet references, inline styles, and the use of !important as potential maintenance or optimization signals.
JavaScript Inspection
Identify patterns such as parser-blocking script references, document.write(), and development-related console statements.
Image Markup Checks
Inspect image markup for missing dimensions and alternative text. These checks do not independently measure image file size or visual quality.
Browser Resource Timing
Review available navigation and resource timing data, paint entries, and observed layout-shift information from a browser audit report.
Exportable Reports
Export supported analysis results in JSON or CSV format for documentation, comparison, or further investigation.
How to Use the Website Performance Analyzer
- Analyze your source files. Use the available file inputs to inspect supported HTML, CSS, or JavaScript text. You can also paste source text into the relevant input areas if provided by the tool.
- Review the detected patterns. Run the analysis and examine the warnings and suggestions. Treat them as investigation hints rather than proof that a particular resource is causing a slowdown.
- Collect browser timing data. If you want to inspect a live page, follow the tool’s browser-audit instructions. Open the target page, run the supplied audit script in that page’s developer console, and copy the resulting JSON report.
- Import the report. Paste the generated JSON output into the report field—not the JavaScript source code—and run the report analysis.
- Export your findings. Download the supported JSON or CSV report to keep a record of the results and your planned improvements.
Important: The browser audit script must be executed in the target page’s developer console. Pasting the script itself into a JSON report field will produce a parsing error.
Common Website Performance Issues to Investigate
1. Render-blocking JavaScript and CSS
Some scripts and stylesheets can delay the first render when loaded in the critical path. Check whether each resource is necessary immediately, and consider appropriate script loading strategies or critical CSS improvements where safe.
2. Large HTML, CSS, or JavaScript Files
Large files can increase transfer and processing costs. Consider removing unused code, splitting bundles where appropriate, compressing text resources, and reducing unnecessary dependencies.
3. Image Dimensions and Delivery
Specify image dimensions to help browsers reserve layout space. Also review image formats, responsive sizes, compression, and lazy-loading choices. The source analyzer’s markup checks alone do not establish whether an image is oversized.
4. Slow Resource Responses
Browser timing information can help identify resources with long observed durations. Investigate network conditions, caching, server response time, third-party services, and resource dependencies before deciding on a fix.
5. Layout Shifts
Unexpected movement during loading can make a page frustrating to use. Reserve space for images and embeds, review dynamically inserted content, and check font-loading behavior. A single browser observation may not represent every visit.
Who Should Use This Tool?
- Web developers: Investigate frontend code patterns and browser loading behavior.
- WordPress site owners: Review theme, plugin, and page-source patterns when troubleshooting slow pages.
- SEO professionals: Find technical issues worth investigating as part of a broader performance workflow.
- Freelancers and agencies: Create structured reports to support website reviews and optimization discussions.
- Students and beginners: Learn how HTML, CSS, JavaScript, and network resources can affect perceived performance.
Browser-Based Analysis With No API Key
FastTL’s analyzer is designed around local source inspection and browser-provided data. It does not require a Google PageSpeed Insights API key for its core workflow, and the described analysis does not require uploading your source files to a FastTL analysis server.
Live-page timing data is collected by running the audit script in the browser context of the page you are inspecting. The results depend on that browser session, available timing entries, cross-origin restrictions, and the page’s current state.
Privacy note: Only analyze pages and files you are authorized to inspect. Review the actual tool behavior and your browser’s developer-console output before using it with sensitive source code.
Understand the Results Before Making Changes
A warning is not always a performance problem. For example, multiple stylesheets may be intentional, an inline style may be harmless, and a large file may be efficiently cached. Use the report to create a shortlist of issues, then verify each one with repeatable testing.
For a more complete performance assessment, test representative pages under consistent conditions and use specialist browser performance tools where needed. This analyzer is not a complete Lighthouse replacement and does not independently provide full runtime unused-CSS coverage or a complete Core Web Vitals field assessment.
Frequently Asked Questions
Is the Website Performance Bottleneck Analyzer free?
Yes. The FastTL tool is intended to provide a free way to inspect supported source-code patterns and review browser audit data.
Does it need a Google PageSpeed API key?
No API key is required for the local source-analysis and browser-audit workflow described on this page.
Can it analyze any website from a URL automatically?
It does not perform a server-side URL fetch or automatically retrieve every website’s source. For live-page timing data, use the browser-audit workflow on a page you can open and inspect.
Does it provide a complete Lighthouse score?
No. It provides supported source-pattern checks and browser timing information, not a complete Lighthouse audit or a guaranteed overall performance score.
Does it measure unused CSS and Core Web Vitals completely?
No. It does not provide full runtime unused-CSS coverage, and browser observations do not replace a complete Core Web Vitals assessment based on representative field data.
Why does the JSON report show a parsing error?
Make sure you paste the JSON output returned after running the audit script in the target page’s developer console. Do not paste the JavaScript script itself into the JSON field.
Can I export the analysis?
Use the export controls available in the tool to download supported results in JSON or CSV format.
Ready to Investigate Your Website’s Performance?
Start with your HTML, CSS, and JavaScript source, review the available browser timing data, and turn the findings into a practical optimization checklist.
Return to the analyzer