The main killer for performance on macs comes from the compatibility layer. SL uses the compatibility layer which in OS X, has not been updated since 2009 when Snow Leopard (10.6) was released. Apple wants to move people onto the core implementation becaise it's supposidly better from a developer standpoint https://developer.apple.com/opengl/ (you can see the video on migrating here)
However this has created problems for 3D applications using the compatibility profile, essentially Second Life is stuck using this until something is done to change it. And this creates severe performance draw backs which are probably compounded by the poor 3D driver support from Apple in general. However certain features really kill mac performance such as Advanced Lighting Model. If you pull up About Secondlife under help, You'll see the OpenGL version listed as 2.1, this is because the compatibility profile primarily uses this, along with some other extensions from 3.x.
To give an idea of the performance differential, I have a GTX 670 which under Windows, manages approximately 30-60fps in most areas on ultra, depending on various factors of course, under OS X this falls to anywhere from 12fps-25fps, I often have to use SecondLife with Advanced Lighting Model off, and most other settings on high or lower, as opposed to Ultra.
The perfomrance issues on the OS X veiwer really are at this point dire, and something needs doing about it quickly, this problem has been around since Lion and it's only been getting worse as time goes on. Mavericks can use OGL 4.1 which isn't the newest, but it's a lot better than what is currently in use under SL.