UnrealSP.org

I seem to remember an option to rebuild only part of a map. I'm running into some long rebuild times, and it's starting to get annoying when I have to wait 5 minutes for a rebuild when I only do a small bit of work.

Well, if you group things, then you can hide the parts you don't want to rebuild, and select "Only Rebuild Visible" in the Rebuild Options. Don't know if that's what you're looking for though.

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SZ
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Never messed with grouping things before. Is this something I can do after I've already built a significant portion of a map? I'll have to mess around with it - seems like what I'm looking for.

Yeah. Basically, you select the things you want to group together, then go to the groups tab of the Browsers and click on "New group" then give it a name.
BOOM! You've got a group now that you can add or remove actors from at any time.

Groups are also really useful if you've got a lot of overlapping geometry which is on different levels of the Z-axis, as you can hide stuff to make it easier and less cluttered.

(Someone really should make updated tutorials with all this stuff in which are easy to access... maybe on the new USP website?)

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SZ
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I usually just use "show only selected actors" for that, but as far as I know it doesn't impact build times.

Make sure you check off "Only rebuild visible actors" and hide everything you don't need, and you'll rebuild certain sections way faster. Handy if you've completed an area and need to focus on something else. Groups are the shit.