I agree, but the idea of switching to immutable blocks in this case is so you can use auto-threading on the blocks. ie. you're switching to immutable blocks to

I agree, but the idea of switching to immutable blocks in this case is so you can use auto-threading on the blocks. ie. you're switching to immutable blocks to increase performance through threading. If the immutable blocks don't increase performance through threading in excess of the loss from just using mutable blocks, then the whole exercise is wasted. Bolting on immutable blocks into C# is unlikely to have the same effect for readability as designed-around-immutability of Haskel.