I've noticed that when I have Transmission running my network speed becomes very slow. Even if I'm only uploading data (the same is true even if the upload rate is low as 5KB/sec) via Transmission the download speed becomes very slow throughout the computer. It doesn't mattet if I'm browsing using Safari, streaming music in iTunes, if a software is upgrading itself etc they all become slow. As soon as I close Transmission everything speeds up again.
I'm connected through my Airport time capsule.
Edit: This is to clarify some things as mentioned in Tetsujin's response.
I only get this problem when Transmission is running. Even if Transmission is downloading 0 KB/sec and uploading at 3 KB/sec I still have these problems, so it's not because I'm reaching maximum bandwidth.
I'm actually running through a VPN so my ISP should not be able to see that it's torrent traffic (nothing illegal here, I'm using VPN because I like anonymity). Actually Transmission is running through a udp port that I opened via my VPN provider. I've also tried running it through a tcp port, same issues. I tried randomized ports, same issue.
I've tried using other torrent software and I think this actually might be due to Transmission specifically.