Main-thread yielding on React sites

Sites with scheduler.yield() have a good INP most often at 97%, followed by no yielding API at 94% and requestIdleCallback() at 91%. Both isInputPending() and scheduler.postTask() appear on fewer than 50 sites across the dataset. Pages with little JavaScript do not need a yielding helper, so the raw pass rates mostly reflect how much script each page loads.

85.7% of React sites call a main-thread yielding API, while 14.3% run scripts without one. Among the sites that yield, 23% (2,180 sites) use requestIdleCallback() and 63% (6,014 sites) use scheduler.yield().

Yielding APIs appear on pages with heavy JavaScript bundles. Sites with no yielding API load a median of 40 scripts and spend 632 ms in CPU scripting (71 ms Total Blocking Time). Sites using requestIdleCallback() load 85 scripts and spend 1218 ms in scripting, which pushes median Total Blocking Time to 155 ms. Sites using scheduler.yield() load a median of 82 scripts (623 ms scripting). Breaking up those tasks keeps median Total Blocking Time at 28 ms, and 97% have a good INP.

Six in ten scheduler.yield() sites load 66 or more scripts. On those heavy pages, 97% of scheduler.yield() sites have a good INP with a 28 ms median Total Blocking Time. Sites on requestIdleCallback() drop to 94% (208 ms TBT), and sites without yielding sit at 98% (347 ms TBT).

Wix uses scheduler.yield() on 100% of its sites, and 97% of Wix sites have a good INP. WordPress uses requestIdleCallback() on 61% of its sites, and 98% of WordPress sites have a good INP.

PlatformMain-thread yielding on its sitesGood INP

All React sites 9,570 sitesscheduler.yield() 63%requestIdleCallback() 23%No yielding API (none) 14%95%

WordPress 2,342 sitesrequestIdleCallback() 61%No yielding API (none) 32%98%

Wix 5,824 sitesscheduler.yield() 100%97%

scheduler.yield() requestIdleCallback() No yielding API (none) scheduler.postTask()

Does the platform explain the yielding pass rate?

Wix includes scheduler.yield() in its core runtime on 100% of its sites. Wix alone accounts for 97% of all scheduler.yield() sites in the dataset. On WordPress, 99% of sites without a yielding API have a good INP, compared with 97% for scheduler.yield() and 98% for requestIdleCallback().

Why scheduler.yield() resumes faster than setTimeout or requestIdleCallback()

When a script yields with setTimeout(fn, 0) or requestIdleCallback(), the remaining work goes to the back of the task queue. Any queued timer, third-party tracker, or network callback runs before your function continues. scheduler.yield() pauses the current task so the browser can handle input and paint the next frame, then resumes your code ahead of regular tasks. On script-heavy sites (82 median scripts), that holds median Total Blocking Time to 28 ms, against 155 ms on sites using requestIdleCallback().