The checking is part of the way Bittorrent works. Since the BT protocol treats each chunk separately, the client needs to make sure that every chunk has the proper checksum/hash before it "knows" whether a chunk has been downloaded or not.
Usually it happens pretty seamlessly in the background, but for a 60GB download, the program will have to check every bit of the 60GB set aside on the hard drive to see what's been downloaded and what hasn't been. I don't think there are clients that don't do this check when starting a new session, since I imagine it'd still have to check to make sure nothing's been moved/changed/deleted since the last session