Also try going under the System preferences in the mac OS under keyboard and mouse. and make sure that 1) the mouse is detected correctly and 2 that the middle mouse is setup for "Middle Mouse"

Have you tried to re-map the UVs?

Plugins could defiantly cause an issue with slowness. try disabling them one by one. Also what version of maya are you running, perhaps you might need to update with the latest Service pack.. ...