Re:CPU Usage - When and when not?
Let me hopefully add to some of the good information already here...
First, almost all soft-synths are going to have built in effects, especially reverbs. Reverb can be CPU intensive and it is something you may wish to turn off inside the soft-synth. Also, if you have a bunch of different soft-synths with different notions of a reverb space you are in then it may be harder to get everything to hang together regardless of your CPU situation.
One other thing along the lines of effects which is related to above is to take a look at your per track effects, again concentrating on reverb as my example. Make sure you don't put a reverb on each track unless necessary. Instead, create a separate bus for reverb, put the effect on that bus and then route tracks you want reverb on as sends to the reverb bus. WAY more efficient use of your CPU.
I would classify synths in three categories. The first uses the CPU alone to generate sound. Not much of a strain on disk and generally light on memory also. For simple patches like an lfo modulated filter sweep of a square wave you are not going to have problems just leaving the track active and not frozen. Second is a hybrid model of samples plus algorithms. Omnisphere is a perfect example of this. You will use a lot of disk bandwidth if you have large samples and can get crazy CPU useage if you have sophisticated custom modulations. The third type of soft-synth is something like Ivory 2 pianos which has very large samples with lots of depth for realistic piano. Not much of a CPU strain but potentially a memory and disk bandwidth hog.
In many cases you can relieve a lot of strain on your system by doing sample thinning for case 2 and 3 above. Example in Omnisphere is you can dramatically reduce memory use (and system overhead) by giving a range of keys you are going to play for a given patch. Why load up 88 keys worth of sample space if you are playing a single octave of it for example. Ivory allows you to use lighter versions of samples and, in any case, won't load samples unless you play in the range where the samples are. Or so they say...;-) Kontakt libraries definitely would be a candidate for sample thinning. I am not a Kontakt wizard though. Usually, the documenation on the libraries will give you clues to what features of a library are going to be resource intensive.
Just some ideas to consider that may help...
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