Hi Adam
Thanks for the information. We are using PHP5 and PHP7 with the same problem. We have the online users block and we can see the start_session problem when the block is calling pluginfile.php
I can say that it's a very old problem I noticed somewhere around 2012. But haven't find time to investigate, just thinking pluginfile.php is very inefficient or believing that may be our NFS cluster had some strange problems.
So one day I used xdebug with profiler and discovered the problem. It's not related to PHP7 anyway and may be even memcached but just session locking.
As I followed your links, I'm not very confident that the core developers are concerned by that problem.