Computer speed, VST load

What determines VST load? Number of tracks in record? Does it matter if I’m recording silence or loud noise? Seems to me it shouldn’t. Does routing externally to my PCM91 actually uses more native resources than using a UAD2 reverb? I had thought using the outboard reverb for tracking would use less, allow me to use lower buffer settings. Do Cubase EQ’s add to the load? I have my default tracking template set up with low pass filters on every channel. Would disabling them reduce VST load? And what adds to my VST headroom? Faster processor? More cores, threads, whatever they’re called? More memory? My Windows 7 machine is 3.5gHz, 4 cores, each with (I think) 2 threads, 16 GB of RAM. Tracking with the buffer at 128 doesn’t suck, but 64 feels better to me, but with only three tracks recording and my PCM91 for verb I was getting lots of peaks at 64. Should I just accept that 128 is the best I can do or is there anything I can do to improve performance?