Very little doubt that this initial download is messing it up.
Once the UI is loaded, v2 is indeed very low in bandwidth - but I haven’t succeeded to get the UI loaded from the GX over a slow link. After 5 minutes it was still “Loading…” 
Is there a way to preload the UI from local cache somehow and then have that connect to the GX as data source backend?
If my analysis is correct, GUI-v2 loads a UI one time and then has a data stream open with the GX.
I even have a sneaky suspicion that loading GUI-v2 via VRM downloads the UI from VRM and then sets up the data stream to the GX, instead of loading everything from the GX.
While GUI-v1 is a VNC session that loads & refreshes a remote screen.
The first (GUI-v2) might seem the better option, but flunks at the UI download if downloaded from the GX.
The advantage of the VNC session is no UI preload and only screen changes that need to be sent.
Even with the very chatty VM-3P75CT, the “heavy” VNC stream works without a glitch over a slow link.
Going through regular GUI-v1 menus is sometimes laggy over a slow link, but it works just fine.
If GUI-v2 UI can’t be preloaded (via VRM), it’s unusable over a slow link I’m afraid.