Running some benchmark on loop for an hour or so would be a decent way to figure out whether it was a fluke or actual cause for concern. 3DMark or Heaven would work as both have a "loop" mode.
As to what I've been playing, I started, and finished Subnautica: Below Zero (took me 1d7h40min, according to my save file, though I did leave the game running at times)
My expectations were kind of low since there was a lot of negativity around the game. Personally I considered it more of a standalone expansion than a full game, and as a standalone expansion it was definitely decent. I mean, it's more Subnautica, which is good.
Game world definitely feels smaller, which was a common criticism, but conversely it's also much, much more confusing. Early on I died quite a few times due to running out of, ironically, water, because I got lost somewhere in a cave system (and everywhere has cave systems). However, there's now these new plants that give you oxygen, and they're everywhere, so dying by running out of oxygen is actually pretty hard, which, eeeehhhh, I dunno, seems a bit counterintuitive and makes a lot of places a lot less claustrophobic and scary than they would have otherwise been.
Smaller size and the tight cave interiors also means that of the larger vehicles from Subnautica only the Prawn made it into the game. Unlike many I never cared too much for Cyclops (the big sub), I did however miss the Seamoth.
The Seatruck really isn't a good substitute for the Seamoth since without modules it has no storage. Furthermore each module you add makes the thing slower, and, obviously, longer, and thus harder to get around the tighter cave interiors. Combined with quite a few of the modules being really situational, I ended up only using three on the regular: the storage module, the one with the crafting station, and the docking one (so I could lug a Prawn suit around), and given how often I used the Prawn (basically only on land, or to mine) I could have done without that last one most of the time as well.
Which brings us to the land component, Below Zero features quite a bit of surface exploration, and it is kinda "meh". Don't get me wrong, instead of air, you gotta watch temperature (Below Zero is the outside temperature, in Celsius), and the atmosphere is great. You also get access to the Snowfox, some sort of hoverbike you're supposed to get around in. Unfortunately it's just not very usable, for starters it has no storage, you also get knocked off of it by certain enemies, all the damn time (oftentimes you get knocked off immediately after getting back on, multiple times in a row. RRRAAAGGGEEE). Using the Prawn was a much less obnoxious way of getting around, sadly.
So we need to talk about the enemies. They were...disappointing. Most of them are way more aggressive than they were in the original, unfortunately they're also way, way less dangerous. It really doesn't take long before most enemies are just an annoyance rather than a threat (yes, *those* too). This is especially true on land (see earlier Snowfox comment)
Unlike Subnautica, Below Zero has a narrated story and a voice protagonist. In fact, there's two storylines that are not directly related, though they do intersect. The first one is the reason you're there in the first place: finding out what happened to your sister. This one fell really, really flat in my opinion. Supposedly they changed writers somewhere during development and I have no trouble believing it with how this storyline went. Started of pretty strong but totally dropped the ball past a certain point. The other storyline seemed a bit more fleshed out.
I feel I should also comment on the voiced protagonist. I generally prefer the silent type, however for plot reasons, that wouldn't really have worked here. Aside from the character you play reacting entirely differently from what I would have envisioned (really, strong reactions to something should be a player choice, not something forced onto them, it is jarring) it kinda worked better than I expected. I didn't hate it.
I'd give the game a 6/10, it's decent, but the story, land exploration, and lack of sense of danger (plentiful oxygen, most predators being more of a nuisance than a threat) make it so I wouldn't recommend it to anyone who didn't like the first game.
Tl;dr it's more Subnautica, If you liked Subnautica, can get past the story, and don't hate voiced protagonists with the fury of a thousand suns, I'd recommend it, certainly when you can get it on sale. For those who didn't enjoy Subnautica, well, doubt this entry will change your opinion.