We use NewRelic to monitor performance of production applications.
Debugging performance might be the best part of a developer's job. There's a clear, numeric problem. When we fix it, that number improves. We can say things like "We made this 175% better."
There's many established techniques for fixing performance problems. A number of them come "for free" with Rails + Heroku:
- Amazon server clusters
- Asset pipeline
- SQL query caching
A number of them require developer thought:
- Database indexing
- Eager loading
- HTTP caching
Page caching is the heaviest handed technique we have, but if we can cache an entire page and push it into a CDN, that will be the fastest option.