If you have 15 torrents downloading. By the time your 4 torrent tries to pull peers from a tracker it had in common with the previous 3 you get timed out for spamming the tracker then all your torrents STALL after that.
So maybe it might not be a bad idea to space lookups on a per tracker basis to like 10 seconds aparts so you don't get timed out for spamming the tracker.