I have the same problem with Firefox on Ubuntu.

First page load takes 10-30 seconds, and after that Firefox is fast and snappy.

However, my computer at work doesn't have this problem (and runs the same version of Ubuntu, i.e. 18.04.1)

Perhaps I should investigate this better, for instance by looking at system load, memory, and perhaps by monitoring system calls using strace.

I used to have this problem and I remember it being DNS related. It was a while ago and I don't quite remember what I did to resolve it, but it had something to do with disabling IPv6 dns resolution while not using IPv6 in about:config and possibly something else as well.

Yes I think you're on the right track. It must be some lurking network misconfiguration. Firefox itself is not broken in this way in my experience (which is not short).