This question asks about the scalability of Ripple in general, and has an ok answer.
However, it doesn't mention the distributed exchange built in Ripple. How is its current/future architecture designed to cope with scale?
It is designed to be highly scalable.
Offers are aggregated with others offers of the same currency pair in the same direction of the same quality (rate). The ledger has a "next index" operation and offers are indexed by quality, so finding the best offer in an order book is as simple as computing the index for the base of the order book and looking at the next ledger entry.