I don't know whether creating an interface{} has a runtime cost (though it may if the type pointer has to be looked up at runtime?), however there's also a memory overhead: I didn't look at everything but the first two benches (pushfront and pushbach) use `int` values, which are 32 bits. But interface{} is 2 words (128 bits).

This quadruples the memory requirements, meaning the backing buffer may well need either more reallocations, or to make larger allocations & have to span multiple cache lines.