When building in an LXD container on an up-to-date Ubuntu 18.04 with not-hwe kernel I recently noticed that -jN builds bottle neck 100% on fuse, basically using up an entire core while everything else is perpetually waiting on input.
This makes build snaps pretty much unusable for large projects because it makes builds such as KDE’s document viewer okular take more than 30 minutes when it should be less than 4!
I’ve seen this on a server with SSD and 3.5 GHZ Xenon CPU and it looks to be completely reproducible: