Video choppy in editor and export

Hello,

I recorded some 4K videos (25fps) to test my new cam. I then tried to cut them but encountered problems.
Used Shotcut before (lower resolutions, longer ago then) and had no problems at all.

While editing the video was quite choppy. After adding filters like text/rotate it got worse.
So I assumed an issue with my hardware beeing too slow, even I can’t really believe it (Core Duo Q8200, 8gb ram, Nvidia GTX660).

BUT:

After exporting it has exactly (!) the same issue while the test-file from this post Stutter after exported plays really well as it should.

So its not some lag but exactly the same as in the editor as far as I can tell.

Used h.264 High, VBR, mp4

Also, the FPS of the source are detected as 25 as they should.

So since the lag is exactly the same in the editor as in the Player while other 4k (and the source files) play fluently I need help somehow… Seems like a bug to me.

EDIT:
Since I assumed my project file might have a flaw or similar I created an entirely new one with just one clip cut to 3 minutes and no filter at all. Same result. a bit less lag but still there.