Lately (with 6.1.4.4 Windows), it's been super slow for me, too. .I've deleted the entire Cache directory, restarted the PC, and restarted StreamFab several times, but nothing has changed.
The first download (I'm redownloading the "Weeds" TV series at 1080p), quickly climbs to 40mps. When a little more than half of the first episode has been downloaded, it gradually drops to 40k-1mbps, where it remains. Each additional episode slowly rebuilds speed, but tops out at 4-7mbps. Then the process repeats for each other episode. In addition, it's spending an inordinate amount of time processing each episode after the video and audio have been downloaded. As a result, it managed to download only 35 episodes in 8-10 hours.
Note: During this, I ran a Speedtest on the PC, and download speed is/was still normal at 430mbps.