I haven't posted here in a while, but I've been wanting this and I have a few suggestions for some ideas for something similar, that wont effect performance that much.
Say you have the ProfitUI Market Window. In the
ProfitUI Market Window (.jpg), the window can show "Your Sales" and "Your Log" horizontally at the right via the button to the top left, but it wont save unless you set the dropdown box in Window Settings and select "Frame and Title" (i think). If you set it to "None" the window will display Your Sales" and "Your Log" like the Default market window. gm9 has it coded to do exactly this.
Currently this is the only way to save certain settings in-game. I think you should allow us (the UI Modders) to make our own custom presets (like Frame Only, etc.) so the user can pick one that better fits their preferences.
Or/And you could just make a new Property and call it "SaveSettings", if it's set to true, whatever the button/sliders/Dividers/Pages/etc,etc. makes changes to- it will save.
These will cut down on performance issues by not saving the .xml every time a change is made. It will be the UI Developers choice by setting an Object's SaveSettings to true.
or maybe I'm just ranting :P
drago.
Edit: gm9's ideas are good