maybe it doesn't make that much of an impact, but what I did to speed up Planeshift was the following:
- play in windowed mode (not fullscreen) and used 800 to 600 pixels). if you still want to have kind of fullscreen-experience you can use the zooming-feauture of OSX (this has to be enabled in the "keyboard & Mouse Systemprefs -> Mouse/Zoom using scroll wheel while holding..." I set the option to only scroll the screen when the pointer reaches the edge of the visible area). What it does, basically, is that when I press the ctrl-key and use my scroll-weel it zooms into the whole screen. I do this until the Planeshift-Window fills the visible area as much as possible.
- turn most of the music off (apart from the sounds when buttons and such things are clicked)
- when in cities I use a camera-distance of around 30 to 40. The sight isn't that beautiful, but at least I can walk around in a fairly good speed. (in wilderness it is no problem to set it to around 150, maybe even more, but this is good enough to find your way)
- as someone else suggested, I set the framerate as high as possible and it really made a difference.
Hope this helps.
BTW: I am playing Planeshift on an iMac G4 (1 GHz with a 64-MB Gfx-Card (nVidia gForce 4 mX) and with 2 GB RAM