Node.js + mongodb, definitely some not-so-performant pages with a few more db lookups than probably necessary. We also spun up a new Atlas cluster in the same region to rule out the cross data center latency.
That sounds very similar. We also spun up a Mongo instance in us-east-2 and the performance was fantastic. Round trip from the server to the mongo instance was super low and comparable to the Heroku / Mongo times on us-east-1
Bummer you had a different experience.