Singular8ty: It seems that there are a number of various posts on all sorts of forums about improving performance or whatever to fix lag/framerate stutters. None of them work. This is the actual fix.

TL;DR: Run OBS (you don't need to record, just have it running). Trust me, it works.

Other things help for some bits (like the single processor mode should not be the default, so you should absolutely change that - I think this helps with the frame drops when using a portal), but none are complete solutions. Disabling VSync helped, but didn't get rid of it for me (and in at least one older build, it also unlocked the physics rate, which meant ragdolls were instant and projectile weapons were functionally hitscan - this thankfully seems to have been fixed).

However, I happened to run it with OBS most recently and didn't notice any issues. I ran it without OBS and had the lag. If you want a video explanation, see this video by Pixelmusement (I'm not him, but I watch his content).

Yes, this does fix the issue. Yes, it's a core engine problem. It should be a relatively easy fix, though, for the devs. You just need to call timeBeginPeriod at the start of the program (and timeEndPeriod on shutdown).

For those of us without source code access, this is a global (system-wide) configuration. So as long as at least one app sets this appropriately, you're good. OBS does that. So the easiest thing to do for all of us is to just run OBS while playing the game. You do not need to record or do anything like that. You just need to have it running.

Hopefully this helps some people with this (annoying) issue. And maybe the devs can fix the engine.

Singular8ty: I tested it. It's not that. At least not entirely. EVGA Precision sets timeBeginPeriod, too, actually, but I monitored the clock. Without OBS it liked to downclock when I had menus open more than with. Opening menus is also what triggers the issue (usually the inventory is what I found); however, that's not the main issue.

In any case, this is the code to fix 99% of the lag:

#include <Windows.h>

#include <iostream>

int main()
{
timeBeginPeriod(1);
std::cin.get();
timeEndPeriod(1);
}

It works. This is the fundamental bug. Period. OBS just happens to set this (1 isn't the best value, but it's the most extreme to prove a point). (True, there is still a little lag after closing the menu, but it gets back in sync after a frame. OBS also fixes this, and that likely is the GPU clock; however, when people talk about lag in this game, that's not what they're talking about and it's pretty unnoticeable.) What people talk about is lag that sticks and makes the game near unplayable. Those 2 API calls fix that completely. That's it. It's a simple bug with a simple fix. OBS happens to do the best job also because of the clock but compile the above program and you'll be much happier than without, too. Or open EVGA Precision. Or any number of programs that set this to a low value.

Edit: formatting the code better. Does GOG forums support markdown? Probably not....
Edit2: Nope. RIP. And it still doesn't like the leading spaces. I'll just simplify the API call to not check for error.