For people deploying to cloud services, in-cloud latency is all the same no matter who the app/database provider is.
For instance, Heroku and MongoHQ are both using Amazon Web Services, so it wouldn't increase the latency to swap a MongoHQ database instance for a Heroku one.
Well, that's not entirely true. If you're running your own data source through EC2, you can make sure it's in the same availability zone (datacenter), which makes latency very small indeed.
For instance, Heroku and MongoHQ are both using Amazon Web Services, so it wouldn't increase the latency to swap a MongoHQ database instance for a Heroku one.