I don’t know the particular answer to your question but some thoughts.
First I have observed that Flash Projector on Mac runs noticeably worse in 64 bit mode than 32 bit mode in some circumstances. The OS is 64 bit and all code normally runs in 64 bit but if an app has both 32 bit and 64 bit binaries you can tell it to run in 32 bit mode, and Flash Projector is the only app I’ve ever done this for.
64 bit should be faster as the 64 bit architectures of CPUs such as x86 are more capable, with wider busses and registers able to handle more data per cycle, as well as being able to ditch older compatibility modes. But as it’s a different architecture it’s possible an unoptimised build will be slower. 64 bit came relatively recently to AIR so it might be it still needs some work.
The code you are using to test it is very artificial. A test of more real-world code, such as an app, would be more useful for overall performance. But what you’ve done seems to highlight a particular problem in a nice self-contained way, so is worth submitting to Adobe.