It seems that anytime you hit F5 in the IDE, the JIT is running in non-optimized debug mode:

You see, whenever you do an F5 from inside the IDE, the IDE figures that you want to be able to debug your code, and therefore the JIT is put into debug mode, and you don't get the fully optimized path.

This happens even if you are building in a release comparison.

IIRC, if you do a CTRL-F5, you don't get this behavior, but it's generally a better idea to do any timings outside the IDE, as the IDE may have other impact on your timings.

So if you're going to do any timings (even ad-hoc ones) do it with a release build from outside the IDE.

Eric's full post