That really depends. Some interfaces have, for example, complexity constraints (these obviously can't be programmatically enforced). The most basic case is "GetThing() gives an
int- i.e., it halts", in which case, the answer would be "No"- both versions of GetThing() halt and return an int.
But many interfaces have implied or expressly stated performance guarantees, either in complexity or in flat time. For example, in the C++ Standard, it's illegal to implement the library with a blocking call except where the Standard expressly permits.