I've shifted to Ubuntu 18.04 LTS from 12.04 LTS recently. It runs unusually slow. Slow boot, logout and power-off processes; window selection/move a couple of seconds after click in it/mouse movement; occasional freezes, slow site loading, need to click on 'Next' button and not taking the command by Enter key after password on more than one site... are some of the symptoms. What could be wrong? 18.04 LTS, GNOME 3.28.2, 64-bit, Pentium(R) Dual-Core, CPU E5200@2.50 GHz x 2, CPU1 at around 24%, CPU2 at around 30%, Memory 83%, SWAP 46%. Thanks for participating.
For such old hardware from my personal experience on about 60 similar machines I can recommend to use normal traditional and fast desktop called MATE DE.
You can install it with
sudo apt install ubuntu-mate-desktop^ and then select its session from login screen.
And of course consider to buy more RAM.
Removing some Snap applications may save some boot time too.
I realise this is an old thread but I recently had the laptop slow to the point of being unusable after a two stage dist. upgrade, 14.04>16.04>18.04. 16.04 was OK but 18.04 was too slow. The condensed story...the giveaway was continuous noise from the HD and an app called baloo_file-extr which I'd never seen before. A few searches told me it was kde/dolphin related, strange, I'd never installed dolphin on this machine. They probably come in via other kde apps such as digikam. I completely removed both dolphin and baloo freeing 65mb. It took a reboot but now performance is back to what I expected. I hope this helps someone else. Another lesson learned, after considering the amount of time to complete each upgrade, and the cruft remaining, I should have "bit the bullet" and done a clean install of 18.04 and take the opportunity to reconsider what apps I really needed to install.