I also encounter the same problem on 14.04 LTS. With 10+ windows open, the system becomes frozen unexpectedly. Once it's frozen, it either becomes totally unresponsive such that nothing works except for the long-press power button or minimally responsive so that I can shift to terminal using CTRL+ALT+F1. But then, the terminal is so slow as well and I eventually turn the system off with power button.
I also encounter the same problem on 14.04 LTS. With 10+ windows open, the system becomes frozen unexpectedly. Once it's frozen, it either becomes totally unresponsive such that nothing works except for the long-press power button or minimally responsive so that I can shift to terminal using CTRL+ALT+F1. But then, the terminal is so slow as well and I eventually turn the system off with power button.