1.Delete Residual Config packages:
go to System->Administration->Synapic package manager, click botton Status to check whether your system has residual config packages.If your ubuntu has the residual config packages,you have to mark the residul config packages for complete removing one by one,and click button Apply to delete them,please see below picture for details:
2.Delete incomplete packages.
open Terminal,and then implement :
sudo apt-get autoclean
this command can delete all incomplete packages
3.Delete isolated libraies.
sudo apt-get install deborphan
check if there are isolated libraies.
If have,please implement below commands to delete them
sudo deborphan | xargs sudo apt-get -y remove –purge
sudo apt-get autoremove
4. Disable unnecessary services
go to System->Administration->Services in order to shut down some unnecessary service, I shut down Bluetooth and Braille display management these 2 services.