This would bolster my ad/tracking script related theory above.
The garbage that those ad companies let through their systems is mostly to blame for the sporadic behavior. A bone stock XenForo is one of the most flexible sites you could ever visit--they test it on many versions of browsers, OSes and devices, and have a very active bug tracking system where the developers are continually checking and asking for feedback from members. It also degrades gracefully, and will still work (albeit with reduced functionality) on older or less capable browsers...even Lynx.
On other forums I've run, many of the problems came from visitors who load up on the ad blocking and JavaScript-crippling add-ons. Well yeah, you're interfering with the site's coding with those dubious programs...we ask members to turn all of that off when troubleshooting. (And
voilá, what do you know--the problem goes away.
) On other forums I've visited, performance issues that existed on that site and not others generally were with the ad networks they used--those things inject JavaScript into the code just to display their ads, and then when some of those bottom-feeding advertisers ram their intrusive ads through the system with their own JavaScript payloads, everything goes haywire. Can't blame a site owner or admin for that, and the only thing to do is to somehow find out which offending ad it is, then have the admin contact the ad network to get them to take it down. The ad networks also may not load asynchronously, so the page may have a delay in loading some of its elements (including JavaScript) as it waits for all of those outside resources (the ad networks) to load.
2. Install an ad-blocker add-on for your browser and see what happens.
Sadly there isn't one for his device, short of using a substandard browser that does not sync up to anything. Chrome, however, has added another "frame buster" mitigation to help with those phony "You have a virus on your phone" ads in their most recent version (Chrome 64.x), so, there is at least some progress for the worst of them. There used to be a way to install an ad blocker after rooting the phone, but my days of doing that with my primary devices have long passed.