need set_max_delay understanding

set_max_delay and set_min_delay are SDC constraints that override existing constraints. They are used if you want to specify to the Fitter a specific range of delay for a net. They should be used sparingly because you are just loosening timing if you choose values for them larger than what is selected based off of the clock constraints you specify.

If you are failing setup timing on internal paths, you should probably not use set_max_delay. There is probably some other explanation for why you are failing timing, such as a long path delay due to physical constraints at the source or destination or you have too much combinational logic on the path. A really good solution is to add pipeline register(s) to break up long paths or logic.