Although I know a fair amount about X, I'm afraid I don't have any good advice for your particular scenario. Sorry!
A couple of notes that might help:
1. As you're aware, you can manually edit XF86Config and hard-code your server to use one or another
display resolution (i.e. hard-code one or another specific "ModeLine"). Perhaps you can test different
settings, and one or another might be more responsive.
2. Quite some time ago, I recall an initiative for "Low Bandwidth X". I don't recall if anything came of it,
I don't know if your system would support it and I don't know if it would be a suitable solution for you.
But here's a link, if you're interested:
3. Finally, here is a neat tutorial that may or may not give you a few tips that you're not already aware of:
Apologies that I couldn't be of more help, and please post back if you find anything interesting.
Your .. PSM