Overlooked reason for debug step slow-down in Visual-Studio

Introduction

In this short tip I want to note a hard-to-find (for me) reason for Visual Studio slowness in debugging.

Description  

Visual Studio has a great debugger - but some things make it go really slow. I won't cover all the top google search results for this... Those tackle problem caused by slow pdb files (e.g. ones on a network drive), by managed code issues, by debug-window evaluations, by exceptions and debug/trace points etc.

It took me time to find it - but for me the slowness was caused by having Tools->Options->Debugging->General->"Require source files to exactly match the original version" unchecked. I just ticked it back and now the step-over speed is back to normal.

What's interesting is that I couldn't find a single reference to this on the web - even after finding the solution. I wonder how many people experienced this...