MacBook Pros have garbage thermals.
Essentially, the fan at the back is unable to effectively move heat sufficiently to sustain any real workload on a MBP, particularly on older MBPs. All machines — laptops and desktops alike — are natural air purifiers, in that they tend to collect dust out of the air. This isn't good for them, and in particular, a heat vent clogged with dust is less efficient at cooling than one not clogged with dust.
On my personal laptops and desktops, this is a relatively minor issue: open the case, blow some canned air over things to dislodge the dust paying attention to the vents and fans, close the case. I.e., you clean it.
MBPs have several issues that compound here, though: first, they cannot be reasonably opened. Only asshats put torx screws into something, to make sure that it is as painful as possible to disassemble, and to force you into an "Authorized Repair Shop". Apple put pentalobe screws in, which is like next-level asshattery. The only reasonable options were flathead or Phillips.
So the case is (unless you own a pentalobe driver, and even my computer repair toolkit doesn't come with one) impenetrable. One might try pusing air through the vents, but this is only so effective. Near as I can tell, the path the air needs to follow is too much.
The next thing that makes this worse is that either the hardware or the OS (I'm not clear on who is responsible for throttling in response to thermals) is much too aggressive. My MBP will often throttle with the core around 65°C. Normally, this is pretty over-eager: CPUs can be run at 80—95°C, so you're giving up performance in order to run the chip cooler here. But, I suspect the reason Apple did this was due to surface temps: when I see aggressive throttling at these core temps, the keyboard's surface temp is also about 105°F to 110°F. (As this is above your body temp, this feels warm, quite warm! It won't burn you, though.) So, while there might technically be thermal "headroom" at the core … taking advantage of it would probably raise the keyboard temp significantly.
(This, to me, is yet another reason that pursuing laptop thinness apparently above all else is folly: this is the "all else": heat management and CPU perf!)
To see an i9 put to this is shameful, really. At the worst, it will throttle to 30%, or 800MHz.
For me, ambient air temps much about 75°F see throttling, above 80°F see significant throttling.
Run:
pmset -g thermlog
CPU_Speed_Limit is the number to pay attention to: AIUI it is the % frequency the CPU is being restricted to due to thermals.
One can try A/C, I suppose.
Companies love these machines, which is … dissappointing, as this is what you get. My own personal preference is Lenovo Thinkpads, though there are plenty of reasons to avoid those, too (the thermals are still not the best, Lenovo has been cuaght putting spyware on their laptops, the design has worsened in recent years) so if you don't like those, I don't blame you. But it shows what a laptop at that price point could be, at the very least. (Even if Lenovo falls short of what it should be.)
Combine that with displaygate, keyboardgate, a meh of an OS, a company actively hostile towards its users, a sub-par warranty, and what seem to me to be inevitable battery recalls (although my current one is still going, somehow, so IDK, maybe battery recalls are behind us) … IDK what companies see in these.