Hi! Thanks!
The reason tall stacks run slow is because there is a large amount of contacts, when bricks overlap (in contact) they need to be "pushed away" from each other (aka depenetration), this is the expensive part. When you blow up the tower bricks go flying and the number of contacts falls significantly, that's why the game suddenly runs smoothly.
The game currently uses 8 cores for physics, and even if spreading to more cores might sound good, the Havok devs told me that there are diminishing returns and spreading to more cores does not seem to improve performance. However they also told me they are working on improving that, so it might've changed in the mean time. The next version will come with the option to set custom number of cores used for physics, so you'll be able to try it yourself.