Keep in mind also some VST instruments (and I'd assume effects also) can be *incredibly* resource-intensive.
A while back I bought and installed Waves' Electric 88 piano. The installation was on my audio workstation which is a so-so machine, core 2 duo processor, 4 gigs of RAM, and an SSD.
When I first fired it up in standalone mode (ie not inside Mixcraft) it used 80% CPU when it was sitting doing nothing at all! Hit a few keys and CPU use would easily hit 100%. The UI worked but was very laggy.
Using another DAW (since the 88 won't work in Mixcraft
), I finally figured out it was the plugin's UI that was consuming most of the resources... with the UI showing it consumed lots of CPU and was laggy, with the UI not displayed it would consume about 15% CPU when idle and 30-40% when playing. Still bad but better.
So I guess to summarize: 1) plugins can use a ton of resources and 2) sometimes whether the plugin's UI is displayed can have an effect.