If the underlying hardware is a bare metal server, it won't magically turn slow and have a CPU spike. That problem is caused noisy neighbor and kind of exclusive to clouds.

Well, with the 2x example, my app might get from a 1s P99 to a 2s P99 which feels slow, but is still doable. Again, those timeouts are usually introduced by cloud infrastructure. For example, if you use nginx outside of Heroku, it won't have a 30s timeout for file downloads.