Well even if I'm not a fan or have ever played a siege map, we have to admit there is a market for them. Otherwise people wouldn't make them

I'm just posting here to keep the HL1 engine from being overstressed, since you do have some serious r_speed and vis issues in your map.
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The first thing you need to fix is in the pillar room behind the door. You have lots of pillars touching the floor and ceiling. This will make the compilers break up the floor and ceiling in more brushes then necesary.
You can fix this in a fast and easy way. Keep a 1 unit grid gap between the ceiling/floor and the pillars.
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Now on to the serious vis problems. The HL1 engine can't deal with very long corridors, even with the low amount of detail you have going it will make the engine stress. This happens because it will have to render beyond it's capabilities due to the lack of vis blockers. To fix this you'll have to make some changes to your layout. Mainly change the long corridors into bendy corridors or something. That is at least the fastest fix there is for this problem.
And with r_speeds going over 900 or even 1200 is just way to much judging from the low detail you're using.