I've tried running the original release of HL2 on two period-correct systems, a 3200+ Barton and a 3.4GHz Northwood, both paired with a Radeon X800 GTO. They both seem to be playable of course, but they hitch on certain parts, leading me to think this is most likely the CPU holding the game back. I've heard before that HL2 benefitted heavily from the Athlon 64 series, should I aim for that platform instead if I'm looking for smoother frame times? I am thinking about swapping to one of my Socket 939 boards while I have my loop drained and taken apart just to give the X800 a bit more of a chance.
I also know that most people weren't getting a solid 60 frames per second back in the day, but tech was evolving fast especially past 2005 when Netburst started to die and AMD retired the Athlon XP. Maybe a CPU one or two years newer than those would possibly give this game a far better chance at running smooth.
This borders on 20 years old, but I think it technically counts lol