My SSD was thrashing and I was running Firefox, Mailspring, and Discord. Since the latter two are Electron-based this was fairly heavy but also fairly normal for me. I had to kill some of those processes occasionally but the SSD still kept thrashing and the computer kept not responding. Eventually I restarted the computer and now it seems to be working for me.
What confused me though was the LXD was taking around 70% CPU (according to top) and I’m pretty sure I didn’t start LXD up for anything (plus top thought it was taking up quite a bit of memory), what could it have been doing and how can I find out if it happens again (or are there any logs I can get to discover what happened last time)? 
Version
$ snap info lxd
tracking: stable
installed: 3.0.0 (6643) 56MB -
$ snap version
snap 2.32.5
snapd 2.32.5
series 16
ubuntu 17.10
kernel 4.13.0-38-generic
$ snap info core
tracking: beta
installed: 16-2.32.5 (4486) 90MB core