I'm wondering if anyone else is using a FireGL V4900 & Windows 8 with 2015. That's what's in my home PC & the lag when sketching is HORRIBLE. I have both 2013 & 2014 on the same computer  & have no lag issues like I'm seeing with 2015. I've checked the Rx & Video driver & I have the current driver  & is the supported driver as I just updated it to see if that would take care of the lag but it is no better than it was with the previous driver. System has 24GB of memory & Rx shows 18.6 GB free with SW running. Any ideas or time to switch to an Nvidia card again.