The issue I had with slow #cargo builds seems to have been because of a disk caching issue. I simply ran; sudo /sbin/sysctl -w vm.drop_caches=3
and now everything's fine!
OK ClamAV On Access Scanning was also to blame, it was set to scan /tmp and /var/tmp which explains why dumping the cache helped. For some reason, a package was having problems getting past ClamAV.
