In digital control systems we can place both observer and system in zero to have a deadbeat performance but there is a Ambiguity in the ratio of performance:
We need observer to be faster than control system because it should estimate states before the control system reacts to values. But if we make both of them deadbeat they will have same performance.
Question: How can we make observer faster without losing deadbeat performance for both parts?
An observer doesn't "need" to be faster than the process. It's a useful heuristic rule, but the separation principle in linear systems shows that the eigenvalues can be chosen independently.
Deadbeat observers and controllers may appear to have desirable properties, but often the practical compromise between speed and noise is not so favorable.