DNS latency is only one factor in overall perceived performance, and it can be affected greatly by routing, traffic load, and other factors. Additionally, many requests you'll make will likely go through a resolver with caching anyhow, so that too can have a significant impact on perceived latency.
Thus DO (or any provider for that matter) only really needs to provide reasonable resolution times from their servers to be effective.
A more important matter is consistency, i.e. are they consistently returning responses or are they actually dropping packets due to load. With a service that's growing quickly, keeping up with the increase in load on their network probably presents a more significant problem.
