I'm assuming you're talking about HL1, not 2 so I've moved the thread.
It could be hanging due to a variety of reasons:
- Have you compilled with RAD and VIS? A map in fullbright will lag
- Entities (particularly NPC's and models) can also cause lag
- Lack of optimisation - for example, use the NULL texture on brushes that the player won't see
Use the Check for Problems tool in Hammer to find out if you've got any invalid errors. Also, check your compile log for any weird errors. Lastly, you are using ZHLT instead of the old Q-Tools, aren't you?