OP had a different issue (see comments further below). They throttled themselves by severely limiting their own upload speed in the torrent client.
That aside you can always torrent without a port forward but the speeds & connectivity will heavily depend on other peers present in the torrent swarm. You’re relying on other connectable peers in the torrent swarm to be able to download torrent data through. If they don’t exist, or there’s not many of them, then you will have performance issues.
e.g. In large torrent swarms you may not notice much difference since there should be plenty of other connectable peers to make direct connections with. The smaller the torrent swarm the less likely you’ll find as many connectable peers which can lead to slower speeds.
And in much smaller swarms without any connectable peers you won’t be able to download anything at all - All the firewalled (not connectable, not port forwarded) peers can see each other but they cannot initiate a data transfer until a connectable peer joins the swarm.