[utensil4778]

I would suspect that the JIT treats a reference as a call to somewhere else in memory, which can have considerable overhead in extremely tight loops. By copying a pointer into a local variable, it may hint to the JIT that you want the pointer in a CPU register with faster access.

Or might be something much more subtle. I know C++ can behave this way, but I don't have in depth experience with C# JIT or x86 assembly.