Thanks again for all the suggestions. I'm a bit puzzled now:
The vnc client clearly isn't taxing the DX's hardware; and the sluggishness is definitely due to the RFB server (but I don't know what to do about it; the -encodings parameters didn't make a difference; I can't find any settings on my vnc server; but anyhow).
Then while aimlessly surfing about, I came across the instructions for the 'Kindleberry Pi' (I have a Raspberry Pi as well; though I haven't tried anything with it yet.) --
wordpress link to the K-Pi. This is basically a recipe for using a Kindle screen as a console for the Raspberry Pi; so, following the instructions, I ran myts on the DX, and ssh'ed into my laptop; then I used tmux to take control of DX's ssh session from my laptop's keyboard.
The author of those instructions has a youtube video, where he's demonstrating, *on a K3*, the refresh rate of the screen while typing. Here's the video:
My tmux session, on the other hand, was just as sluggish as the vnc.
So this makes me wonder, again, whether it's the DX's hardware that is at fault.
(re: Mint/MATE -- thanks for the heads up; I did hear about MATE (possibly on the Linux Action Show), but hadn't tried it. I still like XFCE better
)