Mac is very slow and possibly overheating

My 2019 Macbook Pro (16 GB RAM) is constantly being slow, and the fan is very often running on high which tells me something must be going on.

I've tried everything from scanning it with an antivirus (Avast) to running First Aid and clearing files to make space on the disk, and it still has issues. I've seen similar questions bringing up that at least 50 GB of free space is necessary to run smoothly (which I find very annoying, but there's not much I can do about it). And granted, I am currently only at only around 20 GB of free space, but it was doing the exact same for the past several months even when I freed 100 GB.

Recently, even running discord has been a headache. While scrolling through an unread channel, I'll get the 404 loading screen for a while and my mac will be lagging for a dozen seconds. I'm not able to play a game as simple as Minecraft anymore (even of very low graphics) despite previous, much less powerful Macbooks I'd owned having been able to run it smoothly. I mostly use Opera GX for a browser these days, and opening new tabs is sometimes met with the same amount of CPU (I assume it's CPU) drain. Most of the time, I only having a couple tabs open, granted one of those is generally a paused YouTube video and there might be a social media website like Facebook or Tumblr, but overall, the tabs are inactive and Opera GX is supposed to optimised CPU use.

Note, I am very much keeping my OS up to date. There is currently an update available, but this has been a problem for several months now and no update has solved it so far.

If anyone has any idea how to fix it I'd be very grateful.

Thanks in advance!