Hmmm… If the time to kill was the biggest issue, that’s gonna be something of a recurring problem no matter which Halo you play–one of the things that kinda defines the Halo ‘feel’, as compared with other shooters, is the survivability of both your own character and the enemies you’ll face. For what it’s worth, precision is important with guns like the pistol - headshots bring enemies down quite a bit quicker than bodyshots - and that factor will be further emphasised in later games; Halo 2’s staple gun is the Battle Rifle, which is all about precision at mid/short range.
The fundamental rhythm of Halo’s combat kinda comes down to quickly dispatching the fodder enemies - grunts and jackals, mainly - so they can’t wear you down, then using all the weapons available to you - grenades are particularly crucial - to wear down elites’ shields while maintaining your own. It can be a lot of fun once you get into the swing of things–but plenty of people never warm to it, precisely because it can feel a fair bit slower than other shooters, and that’s just not what a lot of folks want out of the genre, which is fine!
If you’re looking for the good sci-fi bullshit, all the games have it to varying degrees, but the original trilogy is definitely where they do the best job of that classic space opera Bungie Big Ominous Proper Noun world-building–though, really, I think it’s more the tone/aesthetics than anything else that really sells that stuff in those games. ODST operates on a smaller scale, but it might actually have some of my favourite sci-fi bullshit in the whole franchise, particularly in how it handles the idea of AI given body through the municipal infrastructure of a city.
As for Reach, it’s definitely more on the ‘grounded’ military-SF side of things, as @Lilly said, but it does still very much have that wonderful Bungie SF aesthetic - albeit filtered through a grittier, handheld camera war movie-esque lens - that I love so much. Writing that initial post actually inspired me to dig out my copy and give it a fresh playthrough, and I’ve been taking screenshots as I go (sorry for the massive screenshot spam!):