Then it's not the disk. Could you quantify "slow"? If you've left it for a day and it's in 0.4, then there isn't necessarily anything wrong. It could take a couple more days to finish. If you experienced connection issues, then you would have stuck at a particular progress for a very long time.

Downloading would surely go faster without Tor. You can try using a VPN as proxy if hiding this activity from your ISP is needed.