Shock123

dude, a full lobby is obviously full of people, which means your cpu has to render more things, resulting in lower fps. An empty lobby literally has nobody in it, which means it doesn't have to work nearly as hard, which results in higher fps.

or the player count + what they were doing could also affect fps. 
 
I have an idea, play at 100+ ping and going solo like you did here. Then compare that fps in that situation with when you actually had good ping and went solo. 

Well then explain the following... Play in 1440p for best quality as that is the quality of the video. YouTube makes the gameplay look choppy but I assure you it is choppy-free gameplay.
 
Exhibit A...
This is a lobby with 20 people in it. FPS is around 68-80(mostly in the 70's).
 
Exhibit B...
Pretty much the exact same video, only this time, I kicked everyone out the lobby and am now in a solo lobby. FPS is now at 82-103(mostly in the 90's).
 
Conclusion, internet affects fps.

@Shock123 I guess to offer something resembling a solution instead of badgering other people, I found a megathread on Reddit and it appears as Windows kept getting updated, more and more things were added or enabled by default. One of them that seemed to pop up a lot was the Game DVR feature, but considering that feature was implemented back in 2017, I'm not sure how much of an impact it still has today.

Fast enough for gaming? Sure. Fast enough for lag-free gaming? Not even close.
As I have stated over and over, I deal with these types of internet everyday. They lag the game every single time. I used to be in his shoes as well and everyone I played with did not want to play with me because of it. The day I upgraded my internet is the day they no longer complained to me about lag.
I'M NOT MAKING THIS UP AND I'M 100% POSITIVE IT WILL FIX THIS ISSUE.
Sorry for yelling. It's for effect. There's a reason when you sign up for internet packages, the most expensive ones literally have the word GAMING in that package. When you look at the basic packages, the word gaming is nowhere to be found. For a reason dude, for a reason. That very reason has everything to do with lag-free gaming.
 
The best way for me to put this is that while internet is not supposed to affect fps, it most definitely masks or projects something similar to an fps issue, like a lag spike. Exactly what the OP is experiencing. And when you monitor your fps, you will SEE WITH YOUR EYES IF YOU WERE TO OPEN THEM, that the fps is in fact impacted.

It's not a hypothesis, it's a personal experience I deal with every other day. I'm not imagining my fps dip when in full populated lobbies and then rise back up when I kick everyone out the lobby. I'm not imagining lag spikes on my screen when I play online sports games with friends that have similar internet to what you see above. They have yet to upgrade their internet either and playing games like NBA 2k17 with them is very annoying. The lag is exactly as described above when playing with them. And the moment we replace them with a person that has good internet, POOF, the lag is gone.
 
It's not a hypothesis, it's a conclusion.

Or they could try and see if this continues without internet access. If it does, your hypothesis is incorrect.

Upgrade your internet and call it a day. Until then, use MSI Afterburner to show all your temps and usages for your CPU, GPU, RAM, VRAM, etc, etc to show what is going on while you game. Post a clip playing a single player campaign here so we can determine if there's anything going on outside of your internet.

Suit yourself. 100% positive it does, I experience it everyday in GTA 5 online. When I'm in a fully populated lobby, my fps is around 64-82. The moment I get into a lobby by myself, my fps jumps back up to a stable 105. If I just play in story mode, it's at 90-105 consistently.

Exactly what I'm telling you. Upgrade your internet and your fps will remain stable. All the games you listed are played online.
 
Play a game like BF1 offline in the campaign and record the gameplay using Shadowplay and post it here. Be sure to use MSI Afterburner to show everything while you game.

Well there ya' go. Upgrade your internet speeds and the lag spikes will go away.

I don't really think ping or bad internet messes with your frames. Far cry 4, Far cry 5, and battlefield 1 on single player all still have random lag spikes.