I actually haven't tried out the level where they're having the firefight down the street yet, but I highly doubt that it's AI.
It's actually quite painful to play HL2 on my comp, I'm running on a P3 866 with 384 megs of ram... when I'm inside and there's nothing happening, the game runs surprisingly well... but when I'm outside or there's a lot of physics stuff happening, it slows down to a CRAWL. Note the low resolution of the images (640x480), I had to turn it down that low to prevent the game from looking like a slideshow.