I've been mucking around with SPEC on an older XP machine, and I see some places where I might be able to shave a few % points of DSP off in ArcTan, but only by muting features that aren't currently in use, so your results would depend on what controls you're using.
Outside of that, to minimize DSP load while live monitoring, bypass (right click, processing mode, bypass) anything you're not using. This might include:
ZPan, FreeMastering, ZAG, and Limiters. Obviously if you want to hear the affect of, say, the limiters, they would have to be processing.
As mentioned earlier if you're not using Per Track Automation you could delete or mute (right click, processing mode, mute) the preset manager.
Then when you go offline to do measure levels and apply gains/record you will set ZAG, the Limiters, and anything else you're using to processing.
That's about all I can do for you. The majority of the DSP load comes from the FFT/iFFT bidules in Plogue iteself. For Slice and ArcTan we need two FFTs and five iFFTs. You can see the effect of these on the DSP load by drilling down into SLICE or ArcTan and muting them.
I may be replacing one of my XP machines with an lowcost Atom based box like this:
http://www.amazon.com/Zotac-Intel-Mini- ... B003N198UGand if I do I can let you know how SPEC runs (if at all) on that.