Not the OP but I believe efficient means the least number of bytes in this context.

If as a human C compiler you unrolled a loop, that would be performant, but because you're space limited you do not unroll the loop and that is (memory) efficient.

Lets say you had a 6x8 character cell or a really small sprite for graphics, unrolling the loop would make a copy about 2x faster but would cost about 40 bytes or so.

“Performant” is meaningless marketing speak to just mean “it run fast.” Efficient has a more precise meaning