Trying to increase performance of mask attack with a two character prefix

[Plexus]

The strategy to use -m 20 is the right one as -m 20 is around 10-100 times faster than -m 0 if the mask has a static prefix. The reason why this is not in your case is because your prefix is not entirely static. Only 2 of 4 letters are and therefore hashcat is able to get some speed out of it of the remaining 2. There's no other way to speed this up except to hack the kernel and make it use w[1] instead of w[0] for the inner loop iteration.

Sorry for being a pain but what file(s) would I need to edit and is there any resources out there on how to do this?