I installed 18.04 from 17.04 (passing briefly through 17.10, didn't test it and jumped to 18.04) and I am experiencing a high cpu usage on gnome-shell, it takes almost all available CPU time:

$ top top - 06:23:01 up 40 min, 3 users, load average: 3,30, 2,85, 2,09 Tareas: 249 total, 1 ejecutar, 195 hibernar, 0 detener, 0 zombie %Cpu(s): 24,4 usuario, 3,8 sist, 0,0 adecuado, 71,4 inact, 0,2 en espera, 0,0 hardw int, 0,3 softw int, 0,0 robar tiempo KiB Mem : 8059572 total, 3489680 libre, 2041520 usado, 2528372 búfer/caché KiB Intercambio: 9868284 total, 9868284 libre, 0 usado. 5901376 dispon Mem PID USUARIO PR NI VIRT RES SHR S %CPU %MEM HORA+ ORDEN 23946 lealore 20 0 4043344 454724 98808 S 89,1 5,6 5:37.02 gnome-shell 24598 root -51 0 0 0 0 S 30,0 0,0 1:43.28 kidle_inject/2 24596 root -51 0 0 0 0 S 29,8 0,0 1:43.73 kidle_inject/0 24597 root -51 0 0 0 0 S 29,8 0,0 1:43.56 kidle_inject/1 24599 root -51 0 0 0 0 S 29,4 0,0 1:43.16 kidle_inject/3 23769 root 20 0 936456 168344 69492 S 3,0 2,1 0:12.22 Xorg 24154 lealore 20 0 657992 25260 19100 S 1,4 0,3 0:04.05 indicator-multi 23900 lealore 20 0 51196 5724 3932 S 0,7 0,1 0:02.29 dbus-daemon 24659 lealore 20 0 741896 41592 30524 S 0,4 0,5 0:01.01 gnome-terminal- 265 root 19 -1 167208 56740 55524 S 0,2 0,7 0:02.97 systemd-journal

I first used nvidia drivers, then I switched to intel graphics, and still the same behavior. It only stops when switching to Unity shell, but for some reason in Unity I have no sound (!). I saw several reports here and on Reddit, but none provided a solution, not even a clue of what it might be happening. I have no other gnome-shell extension than those that comes with a default 18.04 install, if any.