A plugin can certainly cause Sonar to glitch if has inefficient UI rendering. When I got v1.0 of PSP Lotary, it's screen updating was so CPU intensive that it caused all UI updating from Sonar etc to become sluggish, and resulted in dropouts, clicks etc. It has these animated bars to show how the speakers are spinning, and I don't know what they did but it caused issues. Once you close its UI, things went back to normal.
 
I'm on my new system now, and no longer experience those issues. But I installed Lotary v1.0.1 on my new system, so I'm not sure if its just because the graphics card is much faster, or whether they actually fixed the drawing issues in v1.0.1 (or both).
 
Somewhat related... I've also seen sluggish drawing in Melodyne as well. I think part of the problem is when plugins are developed cross-platform, they tend to port OSX drawing algorithms to Windows, while Windows uses different ways to optimize screen drawing (like clipping regions etc). It's not that one is better than the other, it is just that they need to be different if you want to make it optimized.

I dunno, I upgraded my video card a  while back and noticed a huge improvement in performance. And the one I had was not a bad spec. 

Since I've been running Sonar I switched to AMD video because I found them to run cooler and quieter than the nvidia performance cards.

With modern (or semi-modern) video cards, I would far sooner suspect driver issues (or badly coded plugin GUIs in need of bugfixes) than GPU limitations. Nowadays even the more basic video card models come with at least double the RAM of an entire computer ten years ago.

ampfixer
Since I've been running Sonar I switched to AMD video because I found them to run cooler and quieter than the nvidia performance cards.

Really? The opposite used to be true (as a rule of thumb), though I haven't checked in a while.

I don't think it depends on the brand any more whether a card is good or bad..

I don't know Dr. Av. There is actually quite a war going on with the NVIDIA vs. AMD (with the fanboys of each anyhow) I prefer NVIDIA but that is for reasons outside the scope of this thread. I will say that AMD has supurior onboard cards than Intel does (as NVIDIA doesn't have one) so in that case, yes AMD/ATI is better.
 
Silk has a point though, a lot of the graphcis stuff is now handled by the GPU even in normal applications. Especially if you are using something ike WIndows 8. All that stuff is offloaded to the card so that the CPU doesn't have to think about any of it. That being said, it's not like you're trying to calculate dynamic lighting between the mixer window and the piano roll, so any card created within the last4 years should work fine. 
 
Some DAWs do have "shiny" options to make things look nicer. For example, FL Studio has a "smooth scrolling" mode which makes the program do everything at about 30-60 FPS which DOES LOOK nice and some of the spectral and EQ plugins do have really shiny looking meters and stuff which generates stuff with some kind of particle type of effect. It's not needed, but it looks cool and is still useful.  As it doesn't slow thigns down (or speed things up in some cases) I just leave them on when in FL. I'm sure Sonar has some thigns like that as well too somewhere.
 

I wasn't saying better or worse really, I've had good cards from either manufacturer. Just that AMD cards and chips have traditionally run hotter than NVidia cards and Intel chips. That may have changed recently though, I haven't really kept up.

ViRiX Dreamcore
There is actually quite a war going on with the NVIDIA vs. AMD (with the fanboys of each anyhow)

 
That war's been going on for as long as NVidia and AMD have existed....
 
And far too much of that war involves fanboys recycling third or fourth hand hearsay. No different to the Fender vs Gibson thing really, other than an awful lot of that particular pointless fight to the virtual death seems to involve a lot of keyboard warriors who've never owned an example of either.
 

ViRiX Dreamcore
Silk has a point though, a lot of the graphcis stuff is now handled by the GPU even in normal applications.

 
Not just the graphics stuff, finding ways to use the gpu as a kind of secondary high-speed high-bandwidth co-processor to do all kinds of things has been around for a few years now.
 

I've never heard of Sonar supporting OpenCL or CUDA if that's what you mean. Not very many applications do unless they are already dealing with graphics (like PhotoShop and Vegas).