There's a bad memory leak in gnome-shell which was found & fixed back in March, but the Ubuntu devs still haven't merged the full fix into Bionic for some reason: https://bugs.launchpad.net/gnome-shell/+bug/1672297

Edit: Having tried in on my laptop, installing the gjs + libgjs0g version 1.53.3 debs from http://gb.archive.ubuntu.com/ubuntu/pool/main/g/gjs/ onto a bionic install seems to work fine. Worth trying if this bug is affecting you?