I have a solution using very long case statements which works but is quite slow. I

Didn't tried it but is it possible to use calculations to get the table from say

Min(Substitute(DayRange);"-";"")) so that you can reduce from 60 the number of case statements.

However, I have the feeling the structure you adopted is not appropriate, and that you'd still have slow response time as you're involving cross-related calculations.

I would think it would be faster to split it up with nested cases, that way, you won't keep rechecking things you already know are true:

Case( Daypart01 = "M-F";

    Case( Start01 = "5a";

       Case( End01 = "6a"; Average(Ratings::mf5a);

                End01 = "7a"; Average(Ratings::mf5a; Ratings::mf6a);

                End01 = "8a"; Average(Ratings::mf5a; Ratings::mf6a; Ratings::mf7a));

             Start01 = "6a";

               Case( End01 = "6a"; Average(Ratings::mf6a);

                        End01 = "7a"; Average(Ratings::mf6a; Ratings::mf7a);

                        ...

Thanks for the idea. Nested case statements have sped up the calculations and window refreshing considerably.