As I worked on a project last night, I started to wonder, why do the effects on a project eat up CPU (and I mean a lot of it), even when the project is just sitting there doing nothing (not being played back, being rendered, etc)?
 
I realized how significant this usage was when I had my Sonar project open in the background doing nothing (only a 6 track project mind you).  And the core bars were really lit up.  I was trying to play back some multi-cam video in the editor of Adobe Premiere and there were clearly not enough resources to do it.  
 
All I had to do was bypass the effects in Sonar and the problem went away.  But this lead me to wonder, why are they taking up so much resources if they are not really being used in the moment.   (stopped project just sitting there).  

Because you could at any time enable any number of input echo buttons, or turn a knob on any effect that might make a sound. Some fx even make sound by themselves, e.g. many vintage modeling plugs generate noise. Those are just philosophical reasons, I think it's just the way the program is written. There'd be a LOT of state management to get around it.

Uncheck 'Always Stream Audio Through FX' in Preferences > Audio > Playback and Recording.
 
And if you want to completely stop SONAR using CPU while editing, you can toggle the audio engine off. It's useful to make a keybinding to this function.
 
Per the Ref. Guide:
 
Always Stream Audio Through FX. If any tracks or buses contain active plug-in effects, the audio engine
will be activated and stream silence through the effects, even if the tracks have no audio data or Input Echo
enabled. You will typically only disable this option if you want to conserve a bit of CPU processing if you play a
project that contains lots of empty audio tracks with effects. There are several benefits to having this option
enabled:
Any effect with a “tail” (reverb, delay, etc.) will finish playing when playback is stopped.
Effects will respond to automation envelopes, even on empty audio tracks.
Many plug-ins do not update their UI properly until they receive audio input.
 

I wonder what this does to plugins that actually generate noise? 

I did quick check of the Prochannel Tape Emulator, and if 'Always Stream Audio' is disabled, it only outputs noise/hiss if Input Echo is enabled or the track is playing back. Interestingly, having an audio clip anywhere in the track will enable FX streaming on playback, and it will process even in areas that have no audio. But if there's no audio anywhere in the track, no audio is streamed.

The VST3 spec has the capability to "turn off" FX when not in use. However it's not enough that SONAR's VST3 implementation supports this; the plug-in has to support it too.

Correct, in VST3 SONAR sets the "silenceFlags" in every buffer when nothing is being streamed. The plugin can respond to this and bypass processing. However most plugins do not support this.
 
Otherwise as brundlefly mentioned, by default SONAR streams audio through all effects.