Ok for the slow download, are you on Ethernet or Wifi? If ethernet go into your Settings > Network setup and change the ethernet speed, if its on 100 set to Auto, if on Auto set to 100. Then reboot for good measure.
Another thing you could try is these steps:
The revert to 2.3.7 is in the advanced menu in the diag tool.
Followed by the soft reset with the logs purge if needed (as instructed in that post).
You could also try updating using the diagnostic tool:
If you still cannot update after that then use the diagnostic tool to update. First use the Download Latest button, then use the Restore Previous... button (it can update as well.
Once you are updated I would do a reboot, click advanced then rebuild database and purge logs.
I think once you get updated to 2.3.9, and then do a reboot with a database rebuild and clearing the logs, that should get you back to normal.
