Many Internet Service Providers (ISPs) detect P2P (BitTorrent) traffic and artificially slow it down to save bandwidth.
Cap your speed to about 90% of your maximum to give your drive breathing room. qbittorrent download speed suddenly dropped
Improperly configured limits can cause "network overhead" that kills your speed. it's a hardware bottleneck.
HDD users often see this more than SSD users. If you see "100% Active Time" in Windows Task Manager for your drive, it's a hardware bottleneck. qbittorrent download speed suddenly dropped