"No instance available" despite WOLongResponsePage

I have a wo app where the user can upload image files which then get  
scaled down with ImagMagick. The scaling happens in a  
WOLongResponsePage. Unfortunately scaling large images uses up so  
much CPU that that I still get a "No instance available" message  
every once in a while. Is there any way I can limit the CPU usage of  
the process or tell wo that the regular requests have priority?

Regards,
Will

P.S.: This is btw deployed on Solaris 10.