Pacing... eesh. Pacing in this game seems to be heavily driven by playstyle, especially once you get some hats. The character is actually incredibly mobile, from what I've seen, and can scoot with the best of them (especially with the bat hat) if you're regularly using the dash (especially mid-air). You've definitely got more vertical than horizontal mobility, but you've still got plenty of horizontal mobility available. I've regularly had to kind of consciously slow myself down, because I was going too fast and getting a bit reckless.
Re: monster HP: Most (non-giant) monsters don't take more than four hits if you're keeping up with material (usually very easy, doubly so given higher-tier drops are pretty common) -- many can actually be one or two shot, if you're jonesing well and using your skills (Fire circle doubling arrow damage, magic weapons or 'zerking massively jacking up your melee hits, stuff like that). At least part of this is using the right weapon for the job, though... battle axes are pretty important for dealing with the slower, less mobile/more predictable enemies, ferex, and the magic wands (especially bolt) are ridiculously useful.
Monsters do trend toward relatively static (slimes, mushroom dudes, djinn, etc.) instead of notably horizontally mobile, though. Still, there's usually at least one of a district's critters that's a charger of some sort... probably one of the flying critters. They are a bit slow on average, but that seems to be mostly to keep things sane for the less reflex-capable of players, heh.
The claustrophobic room design is definitely spelunky-ish (especially in those first few levels, iirc.), though the linearity is fairly unique, from what I've seen. Terraria's linearity is more in item progression -- which, admittedly, Magicite currently has as well -- than level design, to the extent anything randomly generated can be specifically designed.