OK, I had a mix of ABC and Spinhenge tasks running. BOINC decided to use 4 cores for the ABC and Spinhenge tasks and gave the GPU 4 tasks. I tried suspending the CPU tasks and BOINC then gave the GPU tasks 8 instances 0.5 CPU each it said. In this scenario the mouse was very laggy.

In both situations described the GPU went to sleep after a period of time and the tasks then took hours to run (one was 7 hours!). I don't know how long this time period was / is because I don't check that box very often, when I do its usually by Logmein.