Yet some more question cause i still need some help figuring this out.

I run:
-3770k @ 3,5ghz (turbo boost to 3,9)
-660TI power edition OC
-16 gb ram (1600)
-Samsung 840 (OS + games located there)
-2TB HDD 7200 RPM, Recordings go there)

1)When i play CS:GO i get roughly 160-320 FPS, awesome.
But when i start recording in the external H.264 codec @ 1920 x 1080 @ 30 fps my
fps drop to around 80-160. Which i already find fascinating (how can i lose a good 100 fps to record at 30 fps?)
But besides that the game just starts to feel laggy. It doens't run smooth anymore. It feel like 30 fps while the meter shows i have 80-150. Can somebody explain this to me? What is going wrong here. Cause this is not only in CS:GO, (a not so graphic hard game), but also in other games that are more demanding

2)How GPU intensive is Bandicam (I just can't find it on the internet). How does it work? When you press the record button the program starts recording your game over the GPU? sends it to the CPU to process and HDD to write it down? Can somebody maby explain this easy to me or give me a link (would be super happy with that ) cause i want to know.

3) What part could I upgrade best on my PC to prevent the lagg? Since i still consider my pc good to play 95% of the games on high / ultra settings so i am kinda lost on why with recording FPS drop so much.

I have almost the same hardware as you. 'Cept for the Ti and SSD. And I have no problems with capturing CS:GO at all.
1) Enable Vsync. You're just making your card burn without any visible improvements.
60Hz monitor = 60 frames can be seen. 120Hz = 120FPS, etc.
2) Are you using optimal Nvidia Experience settings?
3) Is your external X264 codec configured properly?
4) Do you have fast internet connection? I can upload a sample 60fps video.

1) Will do that
2) No, cause i am capable enough to optimize the games myself. The software only looks at your hardware and gives settings for it. Although i prefer to do those myself, since i know what they do.
3) Yes, i followed the Setup through this site, so that should be no problem.
4) Internet speed is 11 Mbps download speed, 1 Mbps upload speed. But that should not matter regarding recordings. But if you want you can show me a sample video.

But it is not that my video's lag. My video's are fine, it is more when i record that the game shows i have 150 fps left but it feel like 30. It feels laggy. The output video's are good.

Yeah. I kinda wanted to upload a 3.5Gig video, but I see that it would take too much time for you to download it.
In any case, I usually use frame limits or vsync when recording so the videocard always has extra juice (also that should decrease the visible lag that is created by recording (if there's such)). X264 as external codec is completely a software encoding (uses CPU). The X264 CUDA can utilize your GPU. Bandicam isn't perfect yet, so some games can get a bit choppy while recording.
Also something less relevant. It's suggested to record the original framerate. As an example I capture Tomb Raider in 30 fps, looked really choppy. Captured in 60, incredibly smooth.

I see what you mean, makes sense. Altough i think you miss my point . I am doing fine with the output video. The recordings are fine but when i record my gameplay just feels .....laggy. even when i have 60-100 frames it just feels like my pc is lagging when playing the game. The output recording is fine. So i don't know why that is occurring? Any thoughts?

I know what you mean. Did you try capturing with vsync? I know it gets a bit choppy, but you'll have to deal with it.