OpenSource:
Does it make a difference (in speed) if a variable ia created at runtime or on top of a sketch?
Variables "on top of a sketch" are created at runtime. Your question doesn't make a lot of sense. Perhaps if you post the code in question and ask if it can be optimized for speed.
Put it another way, when or where the variable is created is not the issue. How many times you access it, and in what way, and what sort of variable it is, are far more likely to be important.