It's probably waiting to compile the scss for Boost. This is a cpu intensive process and different server environments are quicker or slower at completing it. Until the theme is compiled, every page load will trigger this "waiting" when it fetches the stylesheets.
We recently made a number of improvements to this process, but they are still being integrated - in the meantime some things that have a big affect on the speed are: php versions (php 7 is much quicker), disable xdebug if it is enabled.
Finally - you might just have to wait for the process to finish - go have a coffee and come back and see if it progressed to the next page.