Title is pretty much self explanatory, I can't get consistent even with low settings, and my pc isn't crappy either (i7 5500U, Nvidia 840M, 8GB RAM, i know, it's mediocre, but more than capable of running this game)
The thing is, it didn't use to be like this, I used to have 100 fps avg with much better settings.
Suggestions?

Maybe your graphic drivers have been corrupted? Check them / try reinstall.

may be also heat... dusted or it got loose, or it's not sticking properly on GPU .. etc... can be pretty much anything, from SW (malware crypto mining in background, drivers, weird OS like MS windows, ...) to HW (above or even malfunctioning) ...

It's not possible to tell from the description, nothing specific there (unless all of LFS players with similar HW on the same LFS version would experience it, then it would be probably LFS fault)

Downloaded Geforce Experience, updated the drivers but no improvements at all. Malwarebytes and Bitdefender say I'm good. Heat's not the problem either considering the fans spin slowly compared to other games.

"it used to have 100fps"...what happened after that? Maybe do system restore to the time it used to be good. What is your processor occupancy in task manager? And disk usage? Maybe some malware is using your cpu time? Or maybe you have that nice Win10 "100% disk usage" problem?

Quote from Big Daddy :

There are also Bitcoin hacks which use your PC power.
Completely formatting Hard Drive and installing a fresh new Windows would be a good choice.
If the problem still occurs, then it is an hardware issue.

If that was the case, any other game would run badly, but this happens with lfs only.
Well, too bad it seems there's no fix to this. Looks like I'll have to quit LFS, since it's become quite annoying to play with these issues...

Only LFS run bad and you think there is no fix to it? Just be patient and think carefully all possibilities. Try install new copy of LFS in another folder.

fans are low with LFS = may be actual problem, the PC thinks LFS doesn't require that much power, so it scales down frequencies, until it goes down a bit too much, and you get low FPS spike. I haven't seen windows for decade+, so I can't be specific, but try to check for some performance settings, how to enforce full power even for LFS.

Didn't you upgrade something when it started, like better GPU, or newer drivers (which are trying to save power more aggressively)?

Quote from Ped7g :

try to check for some performance settings

Good point. I have noticed in many modern laptops the default power setting in Windows is "balanced" instead of "maximum performance" even when charged.

Quote from Ped7g :

fans are low with LFS = may be actual problem, the PC thinks LFS doesn't require that much power, so it scales down frequencies, until it goes down a bit too much, and you get low FPS spike. I haven't seen windows for decade+, so I can't be specific, but try to check for some performance settings, how to enforce full power even for LFS.

Didn't you upgrade something when it started, like better GPU, or newer drivers (which are trying to save power more aggressively)?

It's a laptop, I can't upgrade anything if I wanted lol (aside from RAM and the SSD I installed long ago)

BTW I've done a fresh install and things seem a bit better

Reinstalling the game and keeping the laptop in high performance seem to have stop the dips at 30 fps which is something, unfortunately not the same performance I used to have. But at least, I can keep playing LFS, which is nice hehe

Of course the FPS depends also which track you are on. Blackwood being refactored in 2017 it is the most resource consuming track I think.

Quote from crazyboy232 :

Are you sure you use the right GPU when you play the game? Check in game's settings which grapchics card is used! I somehow had a similar problem and resolved it by reinstalling the windows because it always started the game with intel hd, instead nvidia!

of course it was bound to be the simplest of the solutions, lol. Thank you all anyway for the help provided