hmmm. with coding (i.e. new mod) I'm sure it's possible....
But for a map? hmm. I'm thinking back to the finale of HL2. That seemed to be slowed time... It could have just been hacked in, but maybe... Then again I don't know how they did that anyway, so... :\
Player movement can be slowed down with a player_speedmod entity, and physics prop movement can be slowed down with the phys_timescale (is this right?) CVar, but...
do this to test out .. a bit off topic and it dosnt slow down player
1: start a map
2: change gravity to sv_gravity 0
3: change map with changelevel *mapname*
4: set the gravity when loaded map to sv_gravity 600 (cant remember the normal gravity value but i think its 600)
result: all physics go at gravity 0 and the player act like gravity 600 (you cant jump extremly high)
extremly nice effect on a server becouse when you die your body will keep in the air.
and even funnier : put sv_gravity to -1 instead so things go just a bit up .. that will make players jump like a mad man to get the gun the dead player dropped