I think that's because your VPX system type isn't set as Visual Pinball - change that in PinballX settings and then try.
The core(s) maxing out is probably the cause of the longer load times. As far as i'm aware PinballX already lowers it's CPU priority when a game is launched so not much can be done there. The only thing i can suggest is change the VPX launch parameters to
-minimized /play -"[TABLEPATH]\[TABLEFILE]"
So VP is minimised while launching.. but tbh i can't see that making any more than a miniscule difference.
That might be the table script. Check the Table1_Exit routine - does it have a Controller.Stop line?