I tested as much as I could on 15 domains, various WordPress and WooCommerce websites. I haven’t noticed any positive effects.
The PHP JIT (Just-In-Time) Compiler, introduced in PHP 8, brings significant benefits to numerical processing performance, but only marginally improves the speed of typical PHP applications such as WordPress. JIT allows for direct execution of machine code, which can speed up some operations. However, for WordPress sites, the benefits are limited and may not justify the additional complexity and potential compatibility issues. Therefore, enabling JIT for WordPress is not necessary unless there are specific computing needs.
Yes, 3-5% is very small, so we usually don’t see any difference.
You can install the plugin atec Cache Info to see how many KB that JIT uses. For my websites. It is used from 1kb to 250kb depending on the kind of website.