@jmaple and @gearoid are both right and wrong.
The reasoning is actually that the BES Client CPU usage set to <10% only applies to background relevance evaluation and does not apply during action execution at all. If you are using relevance substitution during actionscript, it is not bound by the <10% CPU usage.
Setting a higher CPU usage for the BES Client may make it more reactive to finding things that are made relevant by previous actions, so it could actually speed things up a bit, but not during action execution.
On windows systems, you can actually set the process priority that the BES Client will execute actions with:
action launch preference low-priority
Example:
http://bigfix.me/fixlet/details/3967
It is a good idea for long running background tasks to be launched at low priority on windows systems, particularly when a user is logged on at the time, or may log in while it is running.