How to disable the power management of the CPU on a MacBook Pro?
Searching the man page for pmset (man pmset)  on macOS 10.11 - El Capitan for anything related to reducing CPU speed yielded no results; though I was able to find an Apple discussion thread dating back to 2008.

I suspect that the user ability to regulate CPU speed was removed at some point.  

That said, the issue is that you have a failing/failed battery and it needs to be replaced.  Your CPU is throttling itself down becuase it has detected low reserves in your battery and is close to shutting down; this is by design.

You didn't mention which specific model you had, but generically speaking, you will have to get the top case replaced.

However, iFixit has some instructions on how to replace the battery without replacing the entire top case.  Warning:  It's difficult and involves at lot of work to release the adhesive holding the battery in place.