When CPU at 100%, loss of ocl performance ?

Hello,

I use ocl v1.00 and I noticed a huge loss of performance when my CPU is also busy.
Few figures :
- CPU 0% -> oclhashcat algo LM is at 800Mh/s
- CPU is at 100% -> oclhashcat algo LM is at 200Mh/s

What can explain this ?

Same behaviour with other algo (NTLM, WPA2, ..)

Thank you.