could be you got a low speed PC adaptor? One of my old phones with bluetooth used to transfer at a very slow speed. Have you tested the transfer rate with different devices, if not and if you can do so. See if the PC is always slow, or the or if the Mac is the slow one, or both!
Actually there's one strange thing with my phone, it will transfer at an average bluetooth speed baring to one of my friends phones were it goes very slowly.
Mac to Mac always seems fastest to me, so it does depend on the device.