Alright, here’s my problem: After playing through the entire game on max settings with little-to-no issues, I reached the (currently) final battle at the teleporter to Xen. While the particle effects were very pretty, I think the Black Mesa team may have gone a little overboard - I was down from about 80 FPS to around 3. I ended up having to turn the settings down to minimum, and even then I was only getting around 10 FPS while looking towards the teleporter. When looking away, I had around 30.
Otherwise, Black Mesa was a fantastic experience. Keep up the good work, but please fix this.