How can I disable Intel Turboboost and Hyper-Threading?

I found a way to do this. It does result in a 40% performance drop.

Apple has documentation that disables Hyper-Threading with the following: Boot into macOS Recovery In Terminal: nvram boot-args="cwae=2" and nvram SMTDisable=%01 Click  and Restart.

To revert this, reset the NVRAM.

Here is the official documentation: https://support.apple.com/en-gb/HT210108

Some of the useful tools / notes for reigning in macOS Catalina 10.15 running on MacBook Pro 16" 2019 monstrosity:

• Intel Hyper-Threading

It appears possible to use Apple's cpuctl to take some of the CPUs offline; it looks like an internal tool supplied by Apple; there's no easy way to know which CPUs are hyperthreading ones (I think they're supposed to be the even ones), so, a safe bet may be to disable half of the upper even-numbered CPUs, to effectively disable 1/2 of Hyper-Threading, e.g., on a 6-core machine with 12 virtual CPUs, we can disable 3 of them rather safely:

sudo cpuctl offline 7 9 11 ; sudo cpuctl list

Can also use the CPUSetter from whatroute.net; which has a setting dedicated to specifically disabling Hyper Threading; which results in all even-numbered CPUs being taken offline.

• Intel Turbo Boost

Turbo Boost is most definitely the most power-hungry and useless technology that cannot be easily disabled on a Mac. It would seem that the Turbo Boost Switcher from rugarciap.com can be made to work on macOS Catalina 10.15 after some troubleshooting, but it's kind of buggy, and the free version keeps requiring you to enter a password each time upon a wakeup.

• Verifying that Turbo Boost got disabled

You can use Intel Power Gadget from Intel to monitor actual CPU frequency; and verify that it doesn't go above the spec, e.g., on MacBook Pro 16" 2019 w/ a 6-core i7-9750H, doesn't go above the regular 2.6GHz up to 4.5GHz Turbo.

• Graphics and Transparency

Go to System Preferences, Accessibility, Display, and turn on all the options to "Reduce motion", "Increase contrast" (incl. "Reduce Transparency"), "Differentiate without color".

Also, go to System Settings, Dock, and select "Minimize windows using:" "Scale effect" (instead of "Genie effect").

• Excessive Heat

You should also use Macs Fan Control from crystalidea.com if you want to be able to use your 2.4 kilobuck laptop on your lap without being extremely uncomfortable from the excessive heat.