Why can't I even watch a YouTube video fullscreen on a late 2017 27" iMac with 4 cores and ram upgrades over hearing and causing kernal_task?

It feels like ever since I bought my iMac in late 2017, the amount of work it can handle has been steadily decreasing. For the first couple years, I could play games online, I could watch 2 YouTube videos at once with no trouble, etc.

After about 2019 or so, the amount of work it can handle has declined so steadily and so drastically that it feels like I'm being strangled by the company into coughing up another $2,700 for a new system to feed their vampiric greed, where they could've easily just provided a reliable long lasting unit for the way-too-much they charged. I payed for every possible upgrade available on it. 4 cores, RAM up grades, 4TB of storage, this thing should at least work well enough to handle a simple io game without overheating practically immediately.

Instead, this machine has slowly degenerated to a point where I can't even watch Netflix and scroll at the same time without the freakin kernal_task kicking on and throttling my entire system into utter uselessness until it cools down enough to be able to hand basic tasks. Forget even attempting to play a simple io game online. That's been all but impossible for the last 2-3 years now, let alone anything even remotely more graphically intensive than that.

The frustration of having paid so much money for what should be a high quality machine, and not even being able to enjoy an effing afternoon on it is maddening. Let alone being forced to resort to random support in the exact place where professional quality support should be being provided.

Does anybody have any ideas what's going on with this, or how to diagnose it?