I would suggest to list the number of slots available per 5-minute period, on the information page (along with runway, airport size, etc. which are already listed). Now I do not mean the "actual" slots available, but rather how many total slots are there per 5-minute period at that airport.
I guess all airports have this "value" assigned in the database, either directly or indirectly (by belonging to a specific slot category, which in turn has its slots listed in another database), but either way display of this information would be only to pull up the data from the database.
Reason: When we see an airport has 30-50-80% of slots available, it is tedious to see how many slots are per 5-minute period offered at that airport. So an airport with 30% slots available but 18 slots per 5-minutes may have actually more slots available than an airport with 60% slots available and 4 slots per 5-minutes.