Well, for starters, many AAA games in Unreal had to have many core functions/classes rewritten from UnrealScript to C++ for performance reasons, where often not every call is virtual. Secondly, UnrealScript is not really a great example, since on-top of Unreal being notoriously on the slower-end of game architectures, and even Epic decided to drop UnrealScript.

And importantly, UnrealScript was designed in the 90's, when memory latencies were far less of problem.

Of course C++ is faster, although that has more to do with being compiled rather than bytecode-interpreted. But even so, we played those games on hardware that's very slow by modern standards, and it was fast enough for competitive PvP, so I wouldn't describe it as "slow" in absolute terms.