Are you talking about when you start the browser? This may be caused by a 'bottleneck' with the incoming data.

Firefox has a lot going on when it is launched; establishing various connections and opening various "temporary transactional" files for user data. Also as add-ons begin "populating" the toolbar with their buttons the launch time will be extended. All that could slow down the loading of multiple home pages. Plus with the first launch each day Firefox "pings" the updates server to look for availability of updates; later in the day launches (I believe) skip that "check" (been awhile since I inspected for that "ping").

On my Firefox 57 installation the add-on toolbar buttons seen to be the last to load. Only 5 extensions installed now, as I am still getting that Profile set up while continuing to use an older version of Firefox for awhile.

For me the UI seems to be "locked", unable to use any toolbar buttons until the last one is loaded. And that is with using the default home page, which is making its own connection to a Mozilla server for "Snippets".

Possibly this is related to launching new content processes. Once Firefox reaches the maximum number of processes, the overhead of launching new ones is not incurred again because new tabs are opened in the existing processes.

To experiment with the number of content processes, see: Firefox's performance settings.