Half Life 2 Performance tip
 

Half Life 2 Update #2 - Fixes for Stuttering Issues


1. Reduce your sound quality to Medium in the game options.

2. Go to the icon where you launch HL2, and open its properties (or go to Steam>Play Games>Half Life 2>Right-click on it and select Properties>Launch Options) and in the Target box add "-heapsize [memoryamount]" (without quotes), e.g. -heapsize 128000 makes the game use 128MB of your RAM. Try not to exceed half of your system RAM with this setting.

3. Go to your \Program Files\Valve\Steam\SteamApps\[username]\half-life 2\hl2\cfg\ directory, and create a new blank text file (right click and select New>Text Document), then rename it to autoexec.cfg. Open this file and copy and paste the following commands into it, then close and save it:

snd_mixahead 0.7
cl_forcepreload 1
sv_forcepreload 1
cl_smooth 0

4. Make certain you run Windows Defragmenter after Steam has registered and updated your system to play Half Life 2. You must run Defrag to ensure smoother gameplay because Half Life 2 is a huge install and though it may take a while it's a vital step.


straight from http://www.tweakguides.com/, more to follow

This site is a collection of information on a problem with Half-Life 2 that has plagued many players of this otherwise excellent game. This problem is characterized by a sudden massive drop in frame rate when turning corners or moving into new areas of the game. These drops are usually accompanied by looping or stuttering sound during the frame rate drop which is how most people become aware of the problem. Aside from these isolated drops in frame rate, the game is otherwise perfectly smooth. These sudden drops kill the immersion and lead to a very frustrating experience.

Below you will find additional details on this problem, some theories, and all responses from Valve on this problem
http://www.blep.net/hl2stutter/