The 1st bar (left) on the performance meter is way HOT.

Have you disabled core parking? Many ways to do it - here's one:
https://bitsum.com/parkcontrol/

I had this exact same thing start happening to me recently. It turned out that the Best Service Engine sample player was set up to only use one core. When I switched its preferences to use multiple cores, the problem went away. Maybe check and make sure that none of your sample players or plugins are set to use only a single core. 

to be honest, I didn't even think core parking was an issue anymore. I thought that was with older versions of windows and sonar. does that really still work ?
 

dan le
Hi:
If you start on bar 1 right away, then can you move the whole project to like bar 3 and see what happens.
dan
 

lol
 
Sorry, but they are talking about the Performance Module bar, not the timeline.
 
And I think something is happening under the hood too, because when I use Fxpansion Tremor (my most cpu hog plugin), even just 1 SI track, if I chose a more cpu hog preset, I can hear some audio drops. This didn't happen before. 128 samples btw.
 
To get rid of the issue, I change my desktop power saving mode to high performance in Windows. Normally I use balanced mode, which works fine for me.
 
Don't know if is Sonar or Windows 10 issue.
 
I'll keep investigating.

It's natural for the 'main' core to be using a lot more processing power than the others, because it's not easy to distribute the work evenly across all of them. An i3 is pretty weak these days sadly, so you may just be reaching the limit of what it can effectively do.
 
You might benefit from increasing any buffers, so that the plugins in each of the secondary threads have longer to operate before they have to rendezvous with the main thread.
 
Also, if there are any plugins that can be split into multiple instances - eg. Kontakt or Play with multiple instruments loaded - consider doing that, to spread the load more evenly.
 
And don't forget that you might be able to freeze certain synths, which takes them out of CPU usage altogether.

I've been running SPlat and AD2 on a Q9300 w/ 8GB RAM, so I'd imagine an i3 should be just fine.  
 
What type of audio interface are you using, which driver mode, and what's your latency set to?
 
Also, you want to check out "dsp latency checker" - it's possible something is driving down low latency performance at a system level (wireless, NIC, etc)