Hi again and thanks for the help,
I've used ModbusTCP now and the input / output rates are much better, if someone's interested:
Reading only = 2,5 - 7,4 ms
Reading and Writing to another pin = 7 - 15 ms
Reading, Processing the measures, and Writing to another pin = 6 - 18 ms
I used Analog Input 2.1 / 2.2 and Analog Output 2.1 for my application