On larger projects, I've had this happen quite frequently. You open a lot of files, set break points, and sometime forget to clear them? Well apprently Visual Studio does not like this. I was debugging today and it was taking like 5 seconds to step through the code, line to line.

I had a hunch that maybe breakpoints were my issue, the file I was debugging, only had one breakpoint, but I went to the debug window, and clicked "Clear All Breakponints" the IDE instantly responded, and it was back to lightining quick again.

File under "stupid" or "obscure".