breakpoint affecting the run speed of a macro

Hi,

I have a macro that seems to be running slowly when left to its own
devices. I did some testing to figure out which particular part of the
macro was being slow (a part that loops through an array and writes
data to a worksheet). If I put a breakpoint immediately before the slow
part then hit F5 to continue execution, it runs much faster (2 secs
versus 30 secs without the breakpoint). This makes no sense to me. Can
someone please explain what precisely is happening here?

Thanks!