Sorry for necro but I have some very recent experience with this. I have a coupla Lego games installed on a flash drive and my daughters play fine via USB 2.0 port on their Sandy Bridge PC. Windows 10 installed on SSD, graphics is HD5770 on a 720p monitor, and game video settings are all set to medium. I know it doesn't answer your Steam backup and Dota2/CSS specific questions but at least it speaks to gaming performance via USB in general.