Instructions like MOVS and STOS used to be slower than the equivalent series of simple instructions, but now they're much faster. Even the more obscure and far less useful BCD arithmetic instructions have been made on-par with a longer series of equivalent instructions[1] so not doing the same for the much more useful BOUND is... unusual. The amount of silicon needed to implement MPX (which includes several new instructions) seems far more than it'd take to make a BOUND decode faster.
Of course a language runtime based on using this instruction needs to capture the exception and handle it, core-dumping need not be the only option at that point.
BOUND does raise an exception if it fails but I think that's a good thing - if code is doing array bounds-checking, OOB accesses are very likely to be a bug (and possible security vulnerability) so aborting execution by default is the right thing to do.