As fast as your code will execute.
The more streamlined your code is, the faster it will loop.
The time of each pass through will vary as different decisions are made, different branches are taken, more complex calculations or indexing is performed, interrupts occur.
Each slow-loop will of course slow down the fast loop during which it happens to occur, because of the extra processing.
What we talk of as the "slow-loop" is only a different branch decision made inside the fast loop.
The streamlined default code probably loops roughly on the order of magnitude of 100,000 times per second.
The clock is 10MHz, so no faster than that
