I didn't think you'd mind if I made this a sticky as it was written in a way that even I could understand it.
Understanding the VIS'ing process can be quite hard, so I'll do my best to try and explain it to you.
Ideally, the game engine doesn't want to have to draw parts of the map it can't see - this is to save on the resources needed to render the game world for the player. To do this, when you make a map, the VIS compiler calculates how to carve up the map, and what area can 'see' into another. for example, if you were stood in a certain spot near a window, you could 'see' the area beyond the window, so the compiler would know that. When you make walls and obstacles out of brushes in the map maker, the compiler treats these like they are solid and can't be seen through. So if you bare this in mind, if you have a map that's very open, there won't be many obstacles blocking your line of sight - so the r_speeds will be very high as the game is having to draw lots and lots of the map all at once, as the compiler has decided that you can 'see' everything.
The Quake 2 engine that DP is based on isn't optimised for open maps, so r_speeds become a very big problem with open large maps.
So, the upshot for you is to:
a) fix the leaks so you can get an accurate measurement of what the r_speeds will be like (at the moment, you can't because the VIS compiler's calculations will all be strange beause of the leaky gaps.
b) look at where you can put some 'VIS blockers'. VIS blockers are just solid obstacles which will block your view, and if done properly should hopefully stop the game drawing parts of the map you don't want it to. These can change the gameplay of your map, however - adding something players will have to jump round, etc... so think carefully about how to use them.
c) redesign parts of your map that are causing VIS problems that can't be solved with VIS blockers. This might mean altering the floorplan of your tunnels, etc... so they don't expose too much of the next area when you are walking down them.
d) using Hint Brushes. Read this for information on Hint Brushing: http://www.gamedesign.net/node/52
for you, I think the mid will cause a lot of problems due to the ice being transparent.
For more info, check http://www.gamedesign.net/node/53
for techniques on lowering the r_speeds. Also, consider taking the water out as that will double your r_speeds.
Hope this helps,
- Dagless MD