Nowadays, most Mac versions of games are ports of the PC version. For Mac ports of PC games, PC gamers can't make that complaint, the Mac port is usually done by a separate company from the PC game and from the PC code after it's been implemented in the PC version.
In almost all cases, I think the Mac version also takes second or third priority to the PC version due to marketshare.
The only thing I can think of is maybe some design work of games and network design of games may take into account that it may need to run a Mac and cause some slowdowns for the PC version.
Can anyone name a new game where the Mac version wasn't a port?