For those of you with dual core CPUs (2 cores) that are getting frame stutter / lag try the following, it might also help those of you with multicore systems who crash/freeze.
Disable Multicore rendering through cvar command
(Since we don't have it working in the advanced video options at the moment)
While in the PVKII Menu, open the console with ~
(if it doesn't open go to options, keyboard, advanced and then check "Enable developer console")
type mat_queue_mode 0 to turn multicore rendering off.
(mat_queue_mode -1 to turn it on which is default)
You can also add this to an autoexec.cfg
Or you can follow the steps below, but it's much easier to do the above.
Try setting hl2.exe's affinity to one core only once pvkii is open, if you don't know how, read below:
(Make sure when you do this, you're not in a server already, you might cause reflective surfaces that use cubemaps to have a pink checkered look and then you'd have to restart the game)
Start PVK II, alt tab out of the game then ctrl+alt+delete and select task manager. (Or start, run, type task manager)
Go to the processes tab, scroll down the list to hl2.exe, right click on hl2.exe and select "set affinity"
After select affinity is selected a dialog box will appear with the two CPUs, uncheck one of them, click okay, close task manager.
You're done for now, maximize PVK II and try playing and see if it lags.
If this works, you'll have to do it each time you start the game.
Hopefully it should fix it for most of you.