CPU Leak in Garage Band?

I am an EWI player using GB as my primary environment and liking it a lot. But I just stumbled across some strange behavior I would like to bring to the community's and Apple's attention. I have been showing off my EWI and what it can do to a few friends. All of my instruments are plugins, of course: The Aria set that comes with the EWI USB, Wallander Wivi Band, SWAM, and a few others. I set up a GB project with 18 tracks empty, assigning one of my instruments to each. What happens to the CPU is quite strange. This is on a Macbook Air, top end for mid-2013: 1.7GHz Core i7, 8GM RAM. OSX 10.10.5.

I launch GB. I start playing the first track, and CPU comes up from a background ~17% to ~35%. Then I leave track 1 and start playing track 2. Simple down-arrow or mouse click and a new instrument - nice. CPU comes up about 7%. Then I start playing track 3. Fine. CPU comes up again, 8-10%. And so on. 8 tracks is the limit. At track 9, GB is running over 90%. The track plays, but it's full of pops and clicks. Quite unusable.

In other words, once a track has been touched, its CPU overhead is permanent. And - this instance of GB has been sitting for 2=3 hours now, completely inactive, and GB CPU is still over 85%, almost the peak it was hitting two hours ago.

One usually thinks of a leak as a memory problem. But this really looks like a CPU leak. What the heck is it doing as it sits with its empty tracks and does nothing.

I guess this is not typical usage. But it seems to me it suggests a serious problem in GB.

Has anyone seen anything like this? Any thoughts?