yes if u use transmission ur devise will slow down and the best way to use transmission is downloading when u dont need to do something else for example when u going to sleep.
and if u want fast download u must check the torrent if it has enough seeders and u can check the preferences to disable maximum downloading at 150!! u can change the maximum peers to be connected to 150 instead of 15!!
and when u click on the torrent u can click the (i) info icon and u can change the priority of the torrent to high instead of middle and also the maximum peers to be connected!!l

THE MUCH SEEDERS THAT FASTER THE DONWLOADS GOES!!!