I finally got the Diagnostics page to load and there I see a warning about the server running on a NFS file system, which might cause it to be extremely slow on file-based sessions. I figure this might be the problem, but after finding how to configure this and setting “session_save_handler = dbtable” in config.ini.php I get no improvement, so I’m guessing this was not the problem. And it seems that the warning may be a little misleading? (System check – NFS filesystem warning)
Other than that, Still extremely slow, though I can now get visitor log data to load eventually on PHP 7.2.10, even though it’s more or less unusable as it takes so long for things to load and the diagnostics screen just time-outs with a 504 Gateway timeout error because it takes too long.