Issues with HTTPS/SSL + slow site loading in Chrome and/or Outlook 2016 Stuck at “Synchronizing Hierarchy” stage when using Cached Exchange Mode

We have a client who runs Vodafone Cube 4G hotspots as their main WAN link, routinely getting 40/40 bandwidth test results. Websites would load properly and quickly, as would downloads, in Internet Explorer, but in Chrome there was a routine 8+ second wait for page loads, with HTTPS traffic displaying CSS errors or not loading at all.

Outlook 2016 would also completely fail to load new email in Cached Exchange Mode, even after creating new profiles, new accounts, deleting offline data etc. The client would get stuck at the “Synchronizing Hierarchy” stage of the sync process, and network activity showed at most 5KB/s going up and down.

We examined everything (VLANs, DNS, MTUs, power cycling all network hardware, new router, disabling IPv6, removing antivirus, flushing WINSOCK etc), but in the end, what fixed the issue was opening up an Administrator-level CMD prompt and running:

netsh interface tcp set global autotuning=disabled