FWIW, I tried a simple test where I loaded 10 instances of Nebula 3 Free (all I have) onto a single audio track with a two bar audio loop stretched out to eight bars.   I chose a different preset in all ten instances, then looped those eight bars and started playback with a 5ms buffer (Focusrite 18i6).    The CPU meter for the first core was high, not surprising for an older Q9300 w/ 8GB RAM, but it played back OK.     When I kicked off my screen capture software SONAR's CPU usage increased just enough to go into the red here and there - not sure why - and while playback was a little glitchy there were no dropouts.
 
I can upload a ZIP file with the CWP, video of playback and the bounced audio for comparison (without the screen capture software going) if anyone's interested.
 

Steve could you tell me which presets you used in Nebula?
 

[Non-contribution really... just observation]
 
I went on a general search for info on this plug-in, but could not find anything specific to its pathing at all. What I did find was posts on just about every DAW forum and even their own about issues with it. It comes across as being "intended" for mastering, but seems that each instance comes online "loaded to the gills." Regardless of "performance," the laundry list of issues I saw came across as "poorly scripted."
 
I actually went back through this post, and realized we are talking X3 Producer here, and the question arose "With all of the plug-ins that come with X3 Pro (which are very capable), why this fixation on a 'mastering' plug?" I guess it just comes across as odd to me... if someone's focus is to make music, they will do so "by hook or by crook." So that continued into "Does one want to make music, or cling dearly to a plug-in that has issues with all DAWs?"
 
It really goes back to my initial post, I guess... if a plug-in is hampering music, it will get kicked to the curb quicker than last week's news.... music drives plug-in choice, not vice-versa. We all make bad investment choices.
 
It seems this plug-in is intended (and scripted) for a mastering bus, so those who are using it for other purposes are running powerful machines, or just lucky.

It is not a mastering plugin.I suggest taking a look at january 2012 soundonsound article for a brief overview of the plugin and its possibilities or even better give it a try yourself there's a free version available.If you can make your recordings sound better why wouldn't you?Making music isn't the problem this is about getting your recordings to sound how you like them this plugin does that for me and many others it just doesn't perform well for me,I reiterate for me, in Sonar but it does in  a competitors product.I've spent much more money and time on Sonar and its addons than I have on its competitors product which runs this plugin well thereby allowing me to mix my previously recorded tracks unhampered .
 
          In my opinion it's not unreasonable to try and find it out why.It makes me wonder if maybe I'm spending money unwisely.People compare features between products all the time it's one of the main uses of the internet and when and if they try products and find greatly differing results I think it's important and interesting to know why,maybe others can learn from it. Isn't that the raison d'etre of a forum such as this?I'm not here to plug a plugin or a Sonar competitor I've tried everything I can think of to find a solution to get it to work with Sonar and every suggestion on this thread,nothing worked I think it's only natural to feel a bit disappointed but you know it's not exactly life threatening I'll simply use the other product. Thanks to all who contributed.

Pict
Steve could you tell me which presets you used in Nebula?
 

Sure: 300 Spartans, 70 Rich Chamber, 70 Small Room, Old Plate 11, 70 Chorus, StateOfLogicMD, Boeing 747 Comp, AlexB DigiTubEx, New Man and EQ_Tape_WarmBus.
 
It was just a completely random selection, with the only intent to pick from the various categories (pre, comp, reverb, etc)
 

Thanks Steve I'll try them out on my system.

No problem.   One thing I was wondering was whether the difference between Nebula Server and Nebula Free is a factor, so perhaps testing these particular presets may help.
 
Also, I've just uploaded the CWP and AVI showing playback on my system to
https://bentley.sharefile.com/d/s337dc4f2a55442da if you want to have a look.
 
I hope you're able to track down something to explain this difference!
 

Steve using your combination in Sonar on 1 track on my machine uses less than a 3rd of the 1st core and about a 6th of the remaining 3 cores which is a very dramatic difference,but the ASIO buffer has to be raised to 1024 to eliminate pops completely.I think I know why,the kernel count is very low on most of those libraries but most of the libraries I'm using have much higher kernel counts from 6 to 11 compared to 1 kernel in 7 of the presets you used the  eq tape warmbus uses 11 the newman uses 5 and the Alexb uses 6 kernels it's when using these higher quality(more kernels) libraries that Sonar chokes at 6 but the other product doesn't until 16 instances with an ASIO buffer setting of 128.The more kernels the higher the processor demand.When I tried on my old duo core 3.06 GHz mac with Logic 9 only 4 instances could be used and  the 1st core was being used to the max and the 2nd core hardly touched.

Oh and I forgot to say thanks very much for taking the time to do the testing and upload the videos
 

codamedia
Does the software come with any support?
Have you contacted Acustica Audio to see if they can make any suggestions?
Have they tested their product with Sonar X3 (or even an older version of Sonar)?
Are other people reporting this problem with them - do they even know a problem exists?
Does Nebula have settings to tweak? If so, don't assume the same settings work between all DAWs.
 
Surely they want their software to work efficiently in any DAW or they risk losing customers.
 
If every VST was performing badly in Sonar then it might be fair to point a finger at Cakewalk. But this appears to be more of an isolated problem - maybe Nebula needs a tweak, not Sonar. (Didn't Waves just release an update late last year related to Sonar compatibility?)
 
Just sayin ...

+1. If you are seeing a drastic difference only with this plugin, your first line of action should be to contact that specific vendor and ask if they have done compatibility testing with SONAR. Perhaps they may be able to offer some support/workarounds regarding this. If they do indeed detect a compatibility issue with SONAR that requires our attention, ask them to contact us via this form and it will be investigated. We also provide free NFR's for companies to test their plugins with our products.
 
The developer support form to be used for compatibility reports is here:
www.cakewalk.com/About/email.aspx?EID=11