port forwarding wont slow down your internet speed unless utorrent is set to use all of your download bandwidth, and if it actually does use all of it. the max download speed it can use can be set in Preferences -> connection at the bottem where it says 'max download bandwidth rate'. you can also set up the scheduler to allow utorrent to use ALL your bandwidth at times when nobody will be using the internet. I have mine set up so the scheduler limits utorrent to 20kb less then my max download at times when i will be home so that i can browse the internet and play internet games without trouble. at times when i wont be home, the scheduler lets utorrent use all my bandwidth.
and although yes, you can get occasional high speeds without the port forwarded, you wont get constant high speeds since every piece of info that utorrent downloads will have to go threw the tracker first, instead of directly going from peer to peer. with the port forwarded, utorrent will be able to directly connect to other peers sharing the file(s) you need so it will be able to download the files faster, get seed info on the file(s) faster to start the downloads faster, and then you can also use DHT to continue to download/seed the files even if the tracker goes down.
as an example, every week i download the newly released episode of an anime series.
it usually has 5k+ seeders and 10k+ leechers when i start to download it. without the port forwarded, it'll usually only download at 15-30kbps, however, with the port forwarded and the speed unlimited it'll download at my max of 180kbps, and in only 15-20 mins compaired to 2 hours.