after searching the web, it seems there is no "out of the box" solution which does exactly this. I´ve read many advices using "counting" IC´s which have parallel output but due to lack of digital pins in my project I need a serial output. - So there you find some parallel to serial converter chips.
Or even suggestions to use a dedicated ATtiny85 or an extra Arduino Chip to do this job and send it via I2C to the main Arduino.
The problem is, I need to count impulses on 2 lines simultaneously. And calculate the Speed from them independently.
I´ve looked into some software "polling" solutions or interrupt solutions. And I can forget the polling as there would be so much delay in the code that it would influence the rest of it. Nevermind that this solution can miss out some pulses resulting in not accurate readings.
And I am not sure about interrupt as they would probably still catch every pulse but still need much CPU power to actually calculate something out of them.
Beside this task the main Arduino should control a TLC5947 Led driver which display should be as fluent as possible, and I doubt it would work alongside some "impulse counting" as this would demand too much power to do.
So I guess I need a "dedicated" solution which just "sends" preprocessed values to the main arduino resulting in very little delay in the main code.
I am happy about your suggestions, whether it is something of the mentioned solutions at the beginning of my post (along with an explanation how to do it) or something I did not even think of.