I'm having a similar issue myself, a significant amount of lag and getting the feared spinning rainbow wheel as well. It's slow enough so that it's practically unusable, and I've ran through the potential list of problems and have come up empty handed. Any help from the Serif staff or fellow members would be immensely appreciated.
System Specs:
Macbook Pro Late 2011 Model
Processor: 2.4 GHz Intel Core i7
16GB RAM 1333 MHz DDR3
I've run files off both my internal HD and an external HD as well, both with the same results. The only setting that I've changed in the preferences of AF itself is the RAM usage limit, which I've set to 16GB up from 4GB. The size of the file I'm working with is around 50MB currently.
I've opened Activity Monitor, and it shows nearly 10GB in my cache, while AF seems to be only using around 500MB of RAM, despite it running quite sluggishly. It seems as if it's "refusing" to use the RAM that is available to it.
Unless I'm missing something, surely these specs are more than enough to handle the program, as it seems other users are experiencing no slowdown issues with much less hardware. Frankly, I'm stumped!