Nope, just not killing everything that doesn't NEED killing.
Its quite simple.
The thing is, the first time I play a game, I don't know precisely which enemies need killing. There's bosses you need to nail like helicopters, antlion guards etc but you don't KNOW that you need to kill them, so if you try to avoid the fights and just rush through you are wasting some of the precious 9hrs.
Then of course it isn't always immediately obvious where you need to go. It took me over 9hrs to complete HL2 but that's not to say I was fighting for 9hrs. I remember in Ravelholm for example I was getting frustrated because I couldn't figure out how to get where I needed to go and was just fighting endless hordes of respawning enemies. In other sections of the game I probably cost myself 5mins here and there by for example taking the lefthand door instead of the righthand door.
Furthermore, for me enjoying a game isn't simply putting my foot down, skipping all the conversations etc. I like to immerse myself in the storyline and for example spent a bit of time practicing with the mavity gun and DOG. In rooms with NPCs I go up to them all and try to engage them in a chat. Bear in mind that that it doesn't matter how astute you are at picking your fights, you still have 'cutscene'/storyline sequences which take the same amount of time no matter if you are you the most badass singleplayer FPS gamers on the planet or a complete newbie. In many cases these are impossible to skip because e.g. a door doesn't open until the end of it.
Could I complete all 3 games in 9hrs? Possibly, but:
a) I wouldn't find it easy
b) I'd be relying on previous knowledge, i.e. having played the game before and knowing how to complete all the puzzles first time, enemy weaknesses etc
c) I wouldn't be enjoying the game as much as I did playing through it normally for the first time