One to note - after upgrading to Monterey and Ventura my 2012 unibody pros (also my 2021) ran lots of "background indexing" and the like for Spotlight, Photos, etc. This took a couple of days on the 2012's with fans on high the whole time. After that they settled down.
Almost as if the hardware is only just managing to run the modern OS
They work very nicely now that has completed, though.
As others have said, this can also be the sign of a faulty/"Replace Soon" battery.
Bear in mind that *any* similarly specced Apple Silicon mac is going to be noticeably faster (even opening windows) than your 2012 if you upgraded. Yes, even the 2020 M1 MacBook Air with the same size SSD and same amount of RAM will be quicker.