Mersenne crowded out

I've been able to run BOINC SETI at the same time as the Mersenne/Prime net program - they split CPU use at about 50%. When BOINC pre-empts SETI for Einstein however, Einstein crowds out Mersenne by using 95-100% of my CPU. It hasn't interfered with any other programs I'd like to keep running the Mersenne if possible. Why is Einstein hogging all the power?