In the last time I had the problem, that my system became very slow and wasn’t usable anymore. Here are the steps I’ve taken to get rid of this problem:

1. Cleanup old packages
   • in the terminal:

     $> sudo synaptic

   • got to  Settings -> Preferences -> Files
   • activate the bullet point Delete downloaded files after installation under Temporary Files
   • press the button Delete Chached Package Files
   • at this point it is a good idea to activate automatic cleanup too
     
2. Remove “ghostfiles” from uninstalled software:
   • still in the Syntics package manager
   • click on status in the lefthand navigation
   • select “Not installed (residual config)” in the left-top select-box
   • mark all entries in the right-top select-box
   • right-click on the selectionc
   • select / click “Mark for Complete Removal” in the appearing context menu
   • click Apply in the top navigation
   • confirm the changes by clicking also on Apply in the summary / confirmation dialog
     
     
3. Cleanup the thumbnail cache for all users over the shell:

   $> sudo rm -f /home/USERNAME/.cache/thumbnails/normal/*
   $> sudo rm -f /home/USERNAME/.cache/thumbnails/large/*
   $> sudo rm -f /home/USERNAME/.cache/thumbnails/fail/*/*
   $> sudo rm -f /home/USERNAME/.cache/thumbnails/xlarge/*
   $> sudo rm -f /home/USERNAME/.cache/thumbnails/large/*
   
   # or just search all files under the thumbnails folder and remove them:
   $> sudo find /home/USERNAME/.cache/thumbnails/ -type f -exec rm {} \;

4. Activate automatic cleanup of thumbnail cache:
   • install cache dconf-tools:

     $> sudo apt-get install dconf-tools

   • run the following command on shell (for every user):

     $> run dconf-editor

   • in the left-hand navigation go to org -> gnome -> desktop -> thumbnail-cache
   • change maximum-age to 30 (days)
   • change maximum-size to 64 (MB)
     
5. relax and feel the speed 😀