Firebase Functions Http Request return cached database

I'm new in Node.js and Firebase Functions, I'll try to be specific for my question.

I have a firebase-database with objects including a "score" field. I want the data to be retrieved based on that, and that can be done easily in client side.

The issue is that, if the database gets to grow big, I'm worried that either it will take too long to return and/or will consume a lot of resources. That's why I was thinking of a http service using Cloud Functions to store a cache with the top N objects that will be updating itself when the score of any objects change with a listener.

Then, client side just has to call something like https://myexampleprojectroute/givemethetoplevels to receive a Json with the top N levels.

Is it reasonable? If so, how can I approach that? Which structures do I need to use this cache, and how to return them in json format via http?

At the moment I'll keep doing it client side but I'd really like to have that both for performance and learning purpose.

Thanks in advance.

via Carles