[piaste]

I've had it happen once, sort of, on a relatively small collection. Several seconds' delay due to the use of foreach, that were actually annoying production users.

The issue was that the collection being iterated over was a non-generic .NET 1.0 DataTable. Using a foreach loop would implicitly box and then re-cast each object, while the for loop directly accessed the correctly typed .Item() and did not need to do that.

Ironically, the body of the loop was a fairly tricky logistics algorithm I had just written, so I had every reason to assume the problem was on the inside. Imagine my surprise when I changed it to a for loop - strictly to access the index and print out some timings - and watched the procedure suddenly become instant...