I think the subject tells all. The connection is via a direct VPN
connection of the remote controlling PC to the LAN of the controlled PC
- so there is no proxy inbetween.

I disabled encryption but that did not help much.

Part of the problem seems to be, that Windows RDC and ZDM 7 remote
control are able to saturate the WAN link during a page refresh,
whereas the ZCC remote viewer only uses 1/4 to 1/2 of the link
bandwidth at most.
W. Prindl