I can't look at your map at the moment, but I'll give you some pointers:
There are two things you need to worry about: polygon count (causes low framerates or "computer lag") and entity count (network lag or latency). To see what your polygon count is, type "r_speeds 1" at the console. Wpoly is the world polygon count. Keep this as low as possible since these polygons will affect the framerate the most. Keep them below 1000 if you can. Under 500 is even better. Open areas usually have the worst wpoly counts because large surfaces get broken into smaller surfaces by the compiler. Also there isn't anything to block the visibilty of details so you see everything at once, making the polygon count high and framerate low. If you want a large, open map, you can use some tricks like snaking the map back and forth (as in brainstorm) so only small chunks of the map are visible at any given time. To better see how the compiler breaks the map up and what is and isn't visible, type "gl_showtris 1". "deathmatch" and "gl_ext_multitexture" must be set to 0 before it will work. Hint brushes allow you to better control how the compiler breaks the map up. You might google around for some tutorials on those.
Epoly is the entity polygon count. Things like weapons and ammo boxes increase this. Epolys render faster than wpolys, so that value can be higher than 1000, but if it gets too high then there are probably too many entities in view. This can lead to both network lag and poor framerates. To remedy this, place your items so that only a handful are visible at any given time.
Sometimes entity placement is not under your direct control. Players and paintballs, for example, are prime sources of network lag because their positions are constantly changing and they make sounds. The best way to reduce this lag is to design the map such that there will only be 2 or 3 people in one area of the map at any given time. You can control this by spawning people in slightly different locations, having multiple paths, and placing items such that people will scatter in different directions after spawning. One thing you definitely want to avoid is chokepoints where there's only one way into a base. You can have all kinds of paths, but if there's only one door into a room to get the flag, then it's kind of pointless. I've tried to reduce the problem by adding gun freeze so people can't just sit at the only entrance and spray nonstop, but most server admins just disable it.
It's really up to the mappers to prevent stale gameplay.
