Ok I made a wall made out of alot of brushes, simple flat wall, so it would be easier to add textures. Then after I added the textures I was wondering if I should start deleting brushes that had the same texture as the one next to it, and expanding it. So my question is, will having a lot of these brushes cause lag. I just finished a map and its brushes are at like 94%, so ya, don't know if it matters or not. I don't think it does, but dont know for sure.

I always delete multiple brushes if they have the same texture, mainly because I'm a perfectionist but also because they put more data in the .bsp ( as blocks are embedded by their vertices in the world, hence when you decompile a map you still get each individual brush ). I wouldn't worry too much about this as it is probably a very small difference. As for performance it wouldn't make much difference, but if you have split the wall up a lot, rendering may take up more processing. Also there is a much greater chance of lighting errors ( too dark or bright at the edges of brushes ).

If you are worried that the brush count is too high and it cuts performance greatly, you can always use culling techniques such as area portals, fog, or good placing of hints. Also the design of the chambers itself will improve performance if you think in terms of visibility.

If you are making a large single player campaign, split the levels up and connect them using a transition. If it is coop, you should only have one chamber set per map, the disassemblers should take you to the next chamber set.

I prefer clean mapping techniques. Any redundant brush should be eliminated.

By eliminated, I mean it should be merged with adjacent brushes. No need to delete them.

The rule of thumb is that you should cut brushes only if you need to modify its shape or change its texture. Otherwise, I wouldn't go about clipping brushes all over your map with no intention of modifying them.

Doing so would increase the BSP filesize unnecessarily, and it would impair performance.

Again, everything we do must have a logical reason behind it.