Physics optimization
It's rare that a map that exists without any physically-simulated objects. How can we keep the performance costs of all these items under control?
Spawnflags
These spawnflags apply to prop_physics, and some to prop_ragdoll.
- Start Asleep
- Suspends all physics calculations until the entity has a force of some sort exerted on it. Its primary use is to avoid the first moments of a map being a slideshow as all of the objects in it 'settle'.
- But of course you do want that settling to happen, or else objects will be suspended in the wrong position. You can escape this catch-22 with the
map_editcommand. - Debris
- Debris objects don't collide with anything but brushes, vastly reducing their cost. They can optionally be 'ploughed through' by the local player with
cl_pushaway_force. 
Note:It can get fiddly to set lots of tiny items as debris. Perhaps a programmer could write some code to automatically do so for models under a certain size when the map spawns?- Motion Disabled
- Does exactly what its name suggests. Unlike sleeping, it can be (and is only) affected with the
EnableMotionandDisableMotioninputs.
phys_constraintsystem
This entity manages a group of interacting physics constraints to reduce processing overhead and keep them more stable. Just link the entities to it and the engine will do the rest.
Network performance
The amount of network data physics objects create can become an issue in multiplayer. To help with this, use prop_physics_multiplayer (In Half-Life 2: Deathmatch maps use prop_physics or prop_physics_respawnable, using prop_physics_multiplayer highly not recommended); it simplifies collision models for complex entities to reduce bandwidth, and makes everything a little bit bouncier to try and hide latency paradoxes.
It also automatically makes smaller objects client-side, which can be overridden with the 'Force server-side' spawnflag if needed.
Note:There have been reports of this entity behaving strangely - revert to prop_physics if you encounter problems.