haXe does some additional optimizations over the Adobe compilers; however, the real performance difference in the Alchemy-based code comes from its original nature as C code: it works with bytes, not the AVM2 type system. You can access these special "fast ByteArrays" from both AS3 and haXe - haXe makes it a little more convenient - but in both cases, you're essentially writing C from a language that doesn't have C's features. (Alchemy has some interoperability features, so it's not a total one-or-the-other equation, at least.)

Rendering is the bigger performance culprit, anyway.