You could change the priority of a process in
Task Manager on the
Processes tab. You would right click and choose
Set Priority. The RRAS service is likely run through
svchost.exe but I don't have a machine up and running to check that. You could go to the service and go to properties to see the path to the executable.
However, you don't really provide enough information to determine if that will actually fix the problem (that you don't describe very well).
What other services are running, and have you used the
Resource Monitor to see if you have an actual bottleneck somewhere?