I’m unable to reproduce this issue via several CPU benchmark tests.
Only the plugins being used in a DAW trigger the throttling. Interestingly, the standalone versions do not seem to have the same problem.
Also I tried to recreate the problem on the same computer running Mojave from an external bootable drive and I can’t get it to break there either. Possibly a Catalina issue?
My computer is well out of warranty. Apple are not usually interested in helping for these kind of issues on older machines in my experience.
Totally lost here.