Thank you all for responses.
Yesterday i was trying some several things. When i boot and start Mepis, it works so fluid and nice, even when i try to maximizing/minimizing or moving windows. But after a while, the system starts to go a little loaded. I have the desktop effects disabled.
I will try the command 'top'. If i do not find any 'big' process, i will try to install the sgfxi script
Some other cuestion. I have a widget on the desktop of CPU monitor. ¿It is normal that, when i enter the desktop, is set at 80%? :S
It can be Firefox? I know that in Windows he consumes too many resources. ¿Any lighter alternative to KDE? I dont like Konqueror neither. ¿Maybe Rekong?
Edit: if it helps, im using Mepis 11, 64 bits version