The paid tier right now offers a couple of things, mainly focused on business administration. In particular, the ability to manage accounts in your organization (remove access once someone has left the company for instance) and priority support.
We want you to use Fieldbook as your database, so we don't limit the number of api calls or books you can have in Fieldbook! (we hate when you run up against an artificial limit and have to scramble to get your product back online). :)
We don't scale as far as we want to right now. For a good experience, we're limited to about 10k rows in a book. Definitely something we want to improve a LOT in the future.
Glad to hear you folks are looking at scaling. I love the idea, and for small stuff the app so far is great. My book with around 2500 rows and 20 columns I find to be significantly slower than I prefer, though. Unclear how much of that is backend versus frontend, but definitely a limiting factor for my use right now. In an ideal world it would feel as snappy as Google Sheets.
Definitely hear you! We are working on some performance improvements right now, if you contact us in app, we might be able to figure out what is happening with your data. 2500 rows and 20 columns should still be snappy!
More here: http://docs.fieldbook.com/docs/plans-and-pricing