10 Second Delay Before Curl POST due to ipv6 (Ubuntu)
What do the logs on Heroku indicate for the time used to process your requests? The logs are accurate with all the apps I've run on Heroku (both dev and production) with respect to time used. Are the heroku logs showing 10 seconds used for each request or is that some timing mechanism on the client side?
I would try removing some part of the code path temporarily to see if you can narrow down what might be taking up the time. For example, you might skip trying to render any complex views but instead render a 200 right from the controller. Next, maybe try commenting out the controller code. Keep taking out various parts until you find what is consuming the time.