I'm sorry if I wasn't so clear in my  goals.
I'll try to explain this better:
I've a miniserver with slow CPU (just 1.5ghz) that i'm using for stream files via samba or dlna, for dvb-t recording and p2p downloading (amule adunanza and torrent).
The problem is this computer has all time an high % of CPU usage, adding motion I get often 100%..

I want to know if is possible reduce CPU usage only during the "not recording time", I mean when it is in Idle, for example by reducing fps while idle and increase to max when recording..

Is all this possible ?

Well if it truly is a CPU resource problem the easiest way is to "nice"
the motion process at startup. ie the shell script that launches motion
has the word "nice" before the motion command. You can arrange all your
processes in terms of the need for real time processing too. The dvb
recorder probably needs the highest priority for example.
I'm using yet nice to get maximum priority during dvb-t recording :) but thanks