PHP 8.0 introduced a new JIT compiler, which theoretically decreases the processing time for PHP-generated content. According to the official instructions, it’s very easy to enable, but it is disabled by default in Debian and requires some slightly different configuration changes to re-enable. Be aware that the Debian maintainer decided to disable it due to reports of segfaults from some users. If you’d like to re-enable the JIT compiler anyway, here’s how:
nano /etc/php/8.2/mods-available/opcache.ini
Change opcache.jit=off to opcache.jit=function (you could also try opcache.jit=tracing, but that is the option that’s most commonly reported to cause segfaults). Also add a line opcache.jit_buffer_size=XM, where X is the maximum number of megabytes of memory you would like the JIT to use for caching. PHP.Watch recommends a value of 50% - 100% of opcache.memory_consumption, but they don’t give much justification. You will need to experiment to find a setting that works for you. (It’s also good to be aware that JIT will make no difference to I/O speeds, which takes up most of the running time of many common PHP scripts.)
Once you’ve made those changes and saved the ini file, reload FPM with systemctl reload php8.2-fpm.service. You can use a helper script such as opcache-gui to check on the status of the JIT cache.
Note, I disabled the opcache JIT on our Debian Trixie server because it was causing regular segfaults on WordPress sites that we host. Other sites seemed unaffected. This was with the stock Debian packaged PHP 8.4.