I've never known an upload to affect your download capability to the extent it is.
I use Avast antivirus and Zone alarm firewall.
Firewall is set up to allow zoom.
I have tried every buffer setting size and get similar results.
I normally have the setting to allow parallel of 8 files.
Similar result but not as bad when using a browser.
Checked the settings in my virgin media superhub 2 and cant see anything there slowing it down