Edit (2024.02.03)
1. What is written in this article will probably no longer apply on newest versions of UUF3P.
2. Great difficulties to understanding of what was written in these articles may be surmounted by patience and perseverance in reading them multiple times.
To be more concise, these articles were based on UUF3P version 2.3.2 and older versions of XEdit which are *very* obsolete now.
If you did not notice yet.
3. With newest XEdit 4.1.5 you can still apply the indexing of *any* mod that contains custom-created entries.
Provided that you know what you are doing.
Thank you for your understanding.
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After cleaning UUF3P (see article How to clean UF3P and get rid of Wooden planks bug in UF3P version 2.3.2 ) you can always do a little trick for better indexing of its custom-created entries (the little bugger is bloated with these!) using FO3 Editor.

1. Open the editor.
2. Right-click on Unofficial Fallout 3 Patch.esm
3. Go to the section "Renumber Form Ids" and expand it.
4. Enter the value 000801 and hit Enter.
5. Wait for FO3 Editor to do its job.
6. Save the task and close the editor.

Better indexing => faster searching through all custom-created entries, one tiny step forward for better performance.

Do not do this for the main esms. You will break your game.
Do not do this for CivisRomanus Unofficial Fallout 3 Fixes. I already did that.
As you probably noticed, I maintained the custom-created entries at minimum-minimorum, when I did not (effectively) found any other vanilla-related solution.

Note:
You can always do this with all your 10000 installed mods. I am pretty sure that some of them may have hundreds of custom-created entries un-indexed.

Happy trogs hunting.