To expand on this, Google execute your JS, but in a slightly odd JS environment (I.e. randomness isn’t random, time might not be correct, etc). They give each site a CPU time budget, and your JS execution eats into this.

If you have 5 pages that take seconds to render you might be fine, if you have 100k pages that take 50ms to render you might be in trouble.

Some of the detail of this is exposed in the webmaster tools that Google provide.