Poor, inconsistent, and false FPS in windowed/borderless mode

[Kibōchen]

Whenever I play a game in a windowed/borderless mode, my FPS gets really bad. It would be at a good 60 FPS, but suddenly it will lag and fall to about 40 FPS or so, and then back to 60 FPS again.
Other times, it would tell me I have 60 FPS when I have jittering all over the place; basically a false reading of my frames per second.
I don't have any of these problems when I go fullscreen, then I would have a constant and stable 60 FPS framerate.
My computer specs are:
GIGABYTE GeForce GTX 1070
Intel Core i7-2160
16 gigabytes of RAM
Anybody know a solution to this?

[Bad 💀 Motha]

Run CPUZ and share the validation in here please, cause that cpu is not correct.

For accuracy testing make sure all forms of vsync/gsync are disabled.

[Kibōchen]

Originally posted by Bad 💀 Motha:

Run CPUZ and share the validation in here please, cause that cpu is not correct.

For accuracy testing make sure all forms of vsync/gsync are disabled.

My bad, I forgot.
Intel Core i7 6700K

[emoticorpse]

I was under the impression this was normal performance behavior. I thought performance was always a little better fullscreen than windowed for games.

[Bad 💀 Motha]

It shouldnt really differ since the game window still has to be the top-most primary active app, if that is in focus with the mouse.

Overall it can come down to the game and it's settings also.

[Snow]

No easy solutions on Windows so far, sorry. Unless it's Kaldaien talking - don't bother reading other answers, and he's the one able to help a bit with that. Special K utility he created got "Flip model presentation" that will override the DWM's flip model and give you borderless performance you're looking for.

Originally posted by Kaldaien:

Present Interval = 1, Backbuffers = 2, Device Latency = 3

Those settings might help, tho keep in mind it's just a workaround and is not guaranteed to work with every single game. For example, Special K especially hates Unreal Engine 3 games and make those unplayable.

Originally posted by Kirito:

Other times, it would tell me I have 60 FPS when I have jittering all over the place; basically a false reading of my frames per second.

Actually, no, those readings are absolutely correct. The monitoring software takes that number from GPU's buffers afaik, so if it shows 60 - GPU did render 60. The problem is after the frames got rendered they can still go missing on the way to your screen. It's Desktop Windows Manager making your life harder. It's got its own logic and works with its own pacing, more often than not giving no crap about when frames are being done. Basically, what DWM is trying to do is to force triple-buffered VSync on every single app. The thing is triple buffering used in DWM and in most games are not the same type. In most games your framerate ends up limited to your refresh rate with VSync, as it uses first-in-first-out queue so it waits till your screen reads the whole picture from your GPU before making a new one. DWM, however, uses last-in-first-out queue, so it starts rendering new picture as soon as the previous one is completed. In borderless mode all the frames rendered by the game have to go through DWM to end up on your screen, as considering game and DWM are not in sync - DWM can simply loose half of your frames by showing every other frame twice.
Special K, as I've said. Give it a try. Some games might also see it fixed if you enable in-game VSync, i.e. League of Legends. Some games work with DWM just fine since the beginning, i.e. Dead or Alive 6. Yet so far there's no simple solution to this as there's no known way to disable DWM on Windows 10. Was possible on Windows 7 tho.

[Dïngus]

I've seen GPUs down throttle when in windowed mode, just like when you alt+F4. You can use afterburner to check this. Pretty sure it's a windows "focus" issue.