Why are HashSets of structs with nullable values incredibly slow?

Question

I investigated performance degradation and tracked it down to slow HashSets.
I have structs with nullable values that are used as a primary key. For example:

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Answer 1

This is due to struct GetHashCode() behavior. If it finds reference types - it tries to get hash from first non-reference type field. In your case it WAS found, and Nullable<> is also struct, so it just poped it's private boolean value (4 bytes)