This question already has an answer here:
My Ubuntu has been running pretty slow, but I don't know why. I have the latest updates and all. I installed Ubuntu i368 on a AMD processor does that effect the speed? Thank you.
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
Very little information to go on here, but I'm going to take a shot at it anyway. Normally when Ubuntu gets laggy for no obvious reason it has to do with gvfs-metadata. This is particularly true if you've been moving files around a bit. It's a pretty easy fix. All you have to do is close Nautilus or whatever file browser you are running, open an terminal and type the following:
then hit ENTER and when you get the prompt back, type:
rm -rf ~/.local/share/gvfs-metadata
The first command stops the gvfsd process, and the second command erases gvfs-metadata file which often becomes corrupt or just too big. Don't worry, though, a fresh new gvfs-metadata file is created as soon as the gvfsd process starts back up. At the very least this does no harm and gives you a bit more speed / CPU usage. The worst case scenario is that you see a bit of improvement. The best case is that this fixes your problem.
If you followed Blind Fish replied you may also would want to consider installing Preload, (adaptive readahead daemon) see here:https://apps.ubuntu.com/cat/search/?q=preload&op= read the description FYI....