timing closure problem.

There are cases where max delay constraints are appropriate to make a requirement on specific paths artificially tighter so that the Fitter will work harder on those paths, but I never use those constraints for performance optimization. It is very likely that there is a better solution.

Start with "Tools --> Advisors --> Timing Optimization Advisor --> Maximum Frequency (fmax)" in Quartus.

If none of the recommendations in the Advisor solve your problem, then refer to the Quartus handbook in Volume 2, Section III, Chapter 8 "Area and Timing Optimization".

If none of the recommendations in the handbook solve your problem, then you might have a case where it is impossible for the design in its current state to have positive slack even if Quartus did the best possible (and Quartus might already be doing the best possible). You might need to add pipeline stages, restructure logic to make it simpler, etc.