Speed from quadrature encoder with LS7366


I am still fairly new to the Arduino world, therefore I have some questions that might sound obvious to you.

I am currently working on a project to measure the speed of a quadrature encoder with an Arduino UNO.
The goal of the project is to measure true speed as low as possible, therefore it's not acceptable to say that anything below X values is equal to zero. I need to know what is my error and deal with it.

I was able to measure speed and use the serial plotter to see the results. But with an high resolution encoder I am rapidly in a situation where the interrupt function use all the time allowed and than the board is not able to calculate a speed anymore.

To solve this problem I am thinking of using an LS7366 buffer

Where I am confused is when they talk about a 40MHz crystal/oscillator do I have add something ?

Secondly, what is the maximum speed to exchange information with the Arduino ?
I think that this delay might have to be taken in account while calculating speed.

Is there a more precise and accurate way to measure time than the micros() function ?

Please do not cross post. Other thread removed.