Sometimes Sonar hangs on me when trying to quantize in the piano roll.  I only have 40 tracks or so but I have 9 VSTs in the synth rack and each one is pretty heavily loaded up with 24-bit sample instruments, using about 9GB of memory.  Doesn't seem to have a problem when there are no VSTs loaded.  Is this a known bug?  Is there a workaround?  
 
Asus motherboard, AMD FX-6350 Six-Core processor, 16GB RAM, Windows 10

Are you just quantizing one track? 

Yes.  One small selection of one track.  

tlshimek
 
Asus motherboard, AMD FX-6350 Six-Core processor, 16GB RAM, Windows 10

Soundcard ?

External USB sound, steinburg UR22.  I'm starting to wonder if it's a VST conflict of some kind.  I'm using EWQLSO Play 64, but there was also a Cakewalk TTS 1 in the rack by accident.

That would not have any bearing on processing data. It's data , not audio.  I quantize a lot of midi with both my ASIO interfaces and asio4all. Makes no difference, it's a rendering of data so therefor depends on the software coding, which we all know works for 99.99% of Sonar users, If it's Sonar then the OP has a corupted install and will need to re install too fix it. The computer and/OS could be to blame too, W10 updates ?  
What does your memory and disk usage show while doing this? 
Have you ever run Latency Monitor? http://www.resplendence.com/latencymon
 
Edit: we posted same time so didn't see your reply.. Can't see how a plug in could interfere with a process but I will ask this- is this a midi track or a instrument track. If your using an instrument track try inserting a midi track and copy the data to that first. I never use instrument tracks and this is one reason. 
 

Task Manager shows high CPU usage by Sonar (40-50%) while it's hung, like it's in a loop.  It doesn't happen with a straight midi file before the VSTs are loaded.  

OK, don't use that VST, move on, There are many that play nicely and it's best to avoid the problem children. 
 
 
An FYI TTS-1 will automatically load when you open a midi file if you do not have any midi output devices selected. That is why you had it in the synth rack. 
I open midi files and use TTS_1 to see what's up then replace each part with better VST's. 

Does anybody else have trouble with EWQLSO VSTs and Sonar?  Also, I just noticed there are more recent versions that the one I am running.  I am on 4.3.5.  
 

The reason I ask is because I had problems using quantize using onboard audio.
And yes, even processing midi uses the soundcard. Did you think it stopped working or something ?
 
To the OP, What's your midi buffer set at ?