I've noticed when running a single torrent it will download at say 100KB/s but "hiding" Tixati with the sys tray icon will immediately increase download speed to the throttle settings (in my case 400KB/s, "showing" the GUI results in the download speed crashing back down.  I can replicate this over and over and looking at the bandwidth graph confirms this.

Admittedly my netbook is marginal spec at 1GHz but cpu load is typcially around 30% (shown or hidden) with 500MB of RAM to spare.

Tree or Tab view, peerlists or not makes no difference.  For me the whole joy of Tixati is all the bells and whistles that it offers and I often get more pleasure out of watching the client perform than the stuff I download so this is a real bummer for me!

Currently runnning Debian testing, xfce.  This also happened under Xubuntu.