First thing to say is that the Mac Pro dual CPU is not officially supported but you don't give CPU details.When entering a city or any other place where many characters are around my FPS drops down to 20-30. Not very well. Using Bootcamp there are no such issues.
Mac Pro 2012
nVidia GTX 680 / 2G
32 GB RAM
256 GB SSD
I have tried the default OS X drivers as well as nVidia's 331 (10.9.2) and 334 (10.9.3). This game is performing poor but I am not sure if it is a driver or (Hero-) engine related issue.
Another issue is the frame rate drop if played in full screen. This will half the FPS. I can get around this by using SIMBL and Maximizer but - mhmm.
When in crafting, quest dialogue etc. the FPS drops down by design but is not directly linked to lag as such.I have the same problem as OP. And when I play on my mac there is a really bad lag all the time. It is worst in the cities when I am crafting, opening mail or selling stuff. Buyback option is used all the time. This does not happen when I play on windows computer on the same wired connection. The time it takes from clicking deconstruct or make something is about half a second before anything happens.
iMac - 27-inch, Late 2012
Processor 3,4 GHz Intel Core i7
Memory 16 GB 1600 MHz DDR3
Graphics NVIDIA GeForce GTX 680MX 2048 MB
Software OS X 10.9.3 (13D65)
Resolution 2560 x 1440
HDD: 1,12 TB Fusion Drive