When I get to a PC I'll try it out. But basically instead of using IComparable directly (so you'd just have a single function call), it used to do something far

When I get to a PC I'll try it out. But basically instead of using IComparable directly (so you'd just have a single function call), it used to do something far more complicated first, but still end up calling the same compare function. For equality I'd expect it to inline a check for null before calling the full equality check. Then a later optimization pass or the JIT could fix it up to be just the null check.