Myself, I have had an issue with load times. I do have an older computer where I play LOTRO some times. I found that if you use a heavily modifed skin that the load times are significantly longer than the default UI. This is because LOTRO has to load all the drawings and XML file before it loads the game. But on a respectable machine, the load times are about the same.
So if you were looking for a change without load times being affected. (If you are on an older computer). Just make a change to the elements that you are looking at most of the time. (The main toolbar, minimap, inventory bags etc.)
Other items like a different BIO screen background you wouldn't really need, You look at that screen maybe once a week
At first I thought this was for LOTRO, but I suppose DDO will handle the custom UI's roughly in the same manner.