Use profile to find out how much time each statement takes.
I ran your code. It is strange that first loop is faster than the second.
Elapsed time is 6.062211 seconds.
Elapsed time is 6.134868 seconds.
However, if you change the loop to be 1000 but the size of a to be 100, the result is different. I think the impact of the f==1 statement is so minuscule that it is hard to evaluate in this context.
Elapsed time is 0.307561 seconds.
Elapsed time is 0.272184 seconds.