It's not Mac related, as I get this on the PC. Really annoying, as it usually happens when I am farming for a while looking for a specific poke, like say...Dratini. One finally shows and then the screen freezes. I do hope the programmers will look into this.
I have discovered a work-around for you: If you are doing as I do, and move/run continually looking for a poke, that if early on in your farming session, hit the items button (no matter what poke is up), and let it load, then it seems to pop up instantly thereafter. So when that Dratini finally does show its face, you won't have a freeze screen to stare at, wondering if you're going to lose your catch.
I only have a small sample of experience on this thus far, but so far that has worked out for me....