All sites should be running PHP 8.0+ by now given PHP 7.4 lifecycle ended late last year. PHP 8.0 will hit EOL later this year and sites should be upgrading at this time.
PHP 8.1 was released in December 2021 and was succeeded by PHP 8.2 in December 2022.
PHP 8.1 speeds-up WordPress 3.5% according to their benchmarks. However it came with a handful of deprecations and backward compatibility breaks to be aware of. See the PHP 8.2 release announcement for some less exciting changes, from the perspective of most WordPress users anyhow.
Major releases always require testing. Those at the front of the line on upgrades may be opening some vendor tickets. In my experience it pays to be in line on these major PHP upgrades, just not necessarily at the very front of it. Doing so gets you the best in performance as well as support cycle longevity.
If you need help upgrading your site(s), contact me.