Same answer as Audit benchmark result - explanation please - #2 by nazar-pc

As described in How much time is given to submit a proof in 3g? - #2 by nazar-pc you have ~4 seconds to audit and prove, so depending on how fast auditing is, the remaining time will be used for proving. In practice since you need to audit within 1 second, you have 3s+ to prove.

No, proving performance doesn’t depend on plot size. The only exception here is when you have more than one eligible solution per slot, but in production network this is quite unlikely to happen, so you can more or less ignore it.