Why do I feel like after flashing my router to OpenWRT with ipv6 enabled, my torrent speed won't max out anymore, it gets about 90% of max speed then dropped to 40-50% when i left it overnight. And turning off ipv6 in my windows system the speed is back to 100% immediately. I am on Unifi 5Mbit package.

Is it because I am using utorrent? I tried googling and seen comments like switching off ipv6 on utorrent for best performance, but not sure about other torrent client.