Wow, got 1600 x 900 HiDPI mode enabled. Really love it. Web pages work well, but the UI lag is pretty crappy . I'm going to experiment with 1920 x 1080 HiDPI.
It's going to be worse... The higher the DPI, the more rendering is needed (2x the DPI selected).
I actually use HiDPI only for day-to-day, but when I play HD videos or full screen content, I rather enable the default resolution without HiDPI.
Also, if you are using two monitors and one is disabled, try to lower the resolution of the disabled monitor to the minimum with no HiDPI. It saves videocard processing power and memory.