What is this thing I've heard about, some kind of "fatigue"? I believe it's "memory fatigue", and if I'm not mistaken, it causes a program to slow down? How quickly does it become noticeable?
The more End's you allow your code to skip over in one "iteration" of your code, the more pervasive the slow-down will be, and the sooner the memory error. The available RAM at the start of the program is also a factor. Weak battery life may also lessen the overall speed, but it doesn't affect the memory allotted to programs.
OK, so that is the only time the program will be slowed down?
A minor detail that's probably irrelevant: it's not that the entire program starts running slower due to memory leaks. The slowdown only affects Repeat, While, and If-Then blocks (not even For( loops — those are fine, although they may contribute to the memory leakage if used improperly).