Maybe use CFF explorer? It works for the Black ICE mod for Hoi3 which is very PC-intensive unless you have a Godly rig. All you do is download it, open CKII's exe file with it (by right clicking), select File Header, then Characteristics I believe, and choose "Always use >2gb of memory" and save changes to play away. I may be wrong on a few aspects of this text, but Lou Baboo has a tutorial on it for Hoi3, but it should be relatively straightforward to do with CKII.
Or you can play with no other windows open while playing, or lower settings, or set the CKII exe to high priority on Task Manager.
Hope this helps a lot!