Why can I rule out my ISP setup?
Easy, every multiplayer game like COD/CS has a serverbrowser like that and NOT A SINGLE game makes any problems.
Why can I rule out my Router?
Ports on my router are all opened so that's also not the problem.
Similar problem in DayZ Mod!
And finally I read about a similar problem in the original DAYZ MOD on the forums, but without any fix/workaround.
EDIT:
While I testet the last time I had a command window open where I pinged google.com all the time (ping www.google.com -t).
While I recreated the slowdown and my internet broke down again it somehow didn't affect the ping program. It just worked and worked and worked!