Torrent download speeds a problem since installing Leopard

I've had a look at some torrent-specific forums, including the Transmission (which is what I use, primarily) forums in my quest to solve this problem but everyone seems to be equally perplexed so I thought I'd ask you nice folks here.

The download speeds I used to get with Transmission were fantastic, however since upgrading to Leopard last month I have just been unable to get anywhere near them, and often the files won't transfer at all, even when there are high numbers of seeders and few leechers. I also get error messages underneath some torrents in the transfer display, such as, 'timed out' or, 'torrent hasn't responded yet'.

Does anyone else have this problem with torrents at the moment, and does anyone else suspect it might be because of Leopard?

I also have quite a bit of bother with browsing the web while downloading torrents (although this was somewhat of a problem even with Tiger). If anyone has any remedies for that, I'd also appreciate any info.

[EDIT] Also, I use a Netgear router at the minute, it's a few years old (but again the speed was fine before Leopard, and remains fine for everything bar torrent downloads). Does using an Airport router really make a noticeable difference to wireless speeds, in anyone's experience?

I've used Xtorrent to get a few indie albums recently and I get great speed with Leopard. So it can't be Leopard.

Are all your ports forwarded? Do you have the Leopard firewall on?

I've forwarded one port for Transmission. It says that the port is open, however when I click 'automatically map ports' it says it has failed to map it. (I'm not even sure what this is for, I was just messing around with the settings in an attempt to fix the problem.) Should I have more than one port open? In my preferences panel there is only one field, for the 'incoming TCP port', so I'm not sure how I would go about setting up more than one forwarded port.

I've noticed that even if I leave my computer and let Transmission do its thing, I can come back and nothing at all will be downloading, yet if I quit and relaunch it, I can get speeds of up to about 60/70kbps -- which is still nothing compared to what I used to get, but it's better than zero. That can't be right.

My Leopard firewall was on, but I thought that might have somthing to do with it so I turned it off and it has made no difference.