I agreed with you initially, but is it really as big of a deal in C#? I'm used to compiled languages where "debug build" means "no compiler optimisations", aka every operation done with a variable is a memory load + store, trivial functions aren't inlined so even trivial accessors called in a loop carry function call overhead, etc etc. But this is C#, so the JIT presumably optimises just as much in a debug build as in a release build?

So in C++ terms, it's really just benchmarking "-O2" instead of "-O2 -DNDEBUG". This seems fine.