LoadingSimple mode
Render-Blocking Resources
Detects CSS and scripts that block the first paint and keep users looking at a blank page longer than necessary.
Why it matters
Even when the server responds quickly, render-blocking files can delay the moment users see anything useful. This directly hurts FCP and often LCP too.
Where to run it
Available automatically in Simple mode.
Can also be explored in more depth from the underlying WebPerf Snippets reference.
Open analyzerWhat you learn from this test
Which files delay the first visible paint
Whether the main bottleneck is CSS, JavaScript, or both
How long rendering is blocked before content can appear
Common fixes
Add defer or async to scripts that do not need to block rendering
Inline or preload only the critical CSS needed above the fold
Delay non-critical styles and scripts until after first render
Review third-party tags loaded in the document head