From what I remember reading, when you install Hitman 2 and run it for the first time, the Legacy Pack won't be immediately playable. I believe you need to run the game once, close it out, and then the Hitman 1 levels will download and install in Hitman 2. [...]
Dropped me right in the same training mission from Hitman S1, I played through it again and definitely noticed much worse performance. And this was the training mission, can't imagine how things would get once stuff starts exploding. I'll fiddle around in the settings a bit and see how it goes.
Rather than go through the challenges in the training mission, I exited out to the main menu, it said "Thank you for your purchase" (I didn't buy anything new, so I imagine that's referring to Hitman S1 I already own) and told me to quit the game to load new content. Now it's updating in Steam, I suppose that's the Legacy Pack levels downloading.