Why is Illustrator throttled to 2% speed when it's in the background?
This appears to be be an Adobe issue; specifically, the code-base for AI is not multi-threaded.

I was able to find a discussion in the Adobe forums that addresses your exact issue - Make Illustrator multi threaded on CPU


  Illustrator performance is awful, its slow and lumbering at all but the most basic operations. It is bound to only a single cpu thread which is ridiculous now in an age of multi core and multi thread CPU's and it has been this way for many years. It cannot handle background tasks and is completely out of parity in function and performance with other Adobe software such as Photoshop and inDesign.


Emphasis mine

Unfortunately, if the code cannot support background CPU operations, there's nothing you can do from your Mac's perspective to speed things up.  As the users in the forum all (begrudgingly) accept, the only solution is to wait for an update from Adobe.

Why this is so

Generally speaking, applications in the background are "paused" - technically, they are given low priority.  What this means is that execution stops until the CPU has an opportunity to execute some more commands; usually during an IO fetch operation.  Managing this process is called "CPU Scheduling" and is something the hardware handles for you - the application just has to allow it (relinquish control).

A common misconception of multi-threading is that things execute faster.  This is not the case, it's just more things happen simultaneously making better use of the CPU resources.  It's like having a single taxi (people carrier) shuttling people between an airport and a conference center versus four doing the shuttling.  They're all going the same speed, it's just now you're moving more.

The application being "multi-thread capable" allows the hardware to manage and schedule CPU time and being that there's more CPU that can be allocated, the background jobs get more resources to get the job done.