I too have been forced to design on mac and test on pc. I always use to regard it as mostly a hardware issue, but that is irrelevant now.
I was hoping with the release of flashPlayer 7 these things would improve.
I hate having to design-down to make something run smooth on the mac just because of the player. Theres also many time using setInterval that it skips the code in one of the intervals if there is heavy _alpha switching, or a decent amount of mc's in a new frame.
I also teach an advanced flash class in a mac lab. All the design classes utilize the mac so we don't have a pc available to test things out on. Making anykind of alpha switching, especially with bitmaps, totally bogs the macs down.
Whenever I mention to people that I develop flash websites and applications one of the first things I get asked is why playback is so slow on the mac.
I still have not seen any technical specifications as to why the pc just rips through swf files.
