I have some experience with an online reservation system.
You say that you have 1 database for each property, 400 in total with the same schema. It sounds like you should have all the properties in a single database. You can then link all your pricing, seasons and availablity etc to each property using your unique property id.
My guess is that you are getting slowed down with all the database connects required for each query. Plus you will be repeating the same query on each database (400 times ouch).