Limit entire system's CPU usage

I can think of at least 3 ways of slowing down the whole Mac.

Turbo Boost

Unless you have a very old Mac (or a very new M1), you can reduce temperatures by turning of Intel® Turbo Boost.

Try Turbo Boost Switcher. You will have to go through some hoops to get it running because it uses a kernel extension. The web site has the required instructions.

Boot arguments

An alternative is to disable some cores in the processor. Though I have not done it myself, you should be able to run this Terminal command (with a suitable number for running cores) and reboot:

sudo nvram boot-args="cpus=4"

Xcode Instruments

An easier way to control cores and threads is to use Xcode's Instruments which has the option to reduce the number of threads (used in a hyper-threaded cpu) or to turn off hyper-threading. It has this preference panel:

Even though it says "Active processor cores", this is really "Active processor hyper-threads" unless you disable what it calls "Hardware Multi-Threading".