I have the same problem as OP, which makes me think that this is not common, and yet it affects some people. If it were common, the app wouldn't exist because it is just entirely unusable.
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.