Where does the pmset CPU_Speed_Limit variable come from, and how to modify it
I cannot find out how to alter the CPU_Speed_Limit variable.

You can't.
It's not a variable in the sense that modifying it will increase/decrease your performance.  It's more of an indicator as to what the limit is currently; it's similar to your battery's current charge status - what it is at the time it was measured.
As correctly stated in the comments, your Mac will limit performance when the battery is missing: Broken battery - downgraded performance
Unfortunately, you'll just have to wait until the replacement battery arrives and you install it to regain full performance from your Mac.