ActiveRecord can generate a ton of Ruby objects from queries. So you track the time it takes for the database to return results, and that may be ~20% of your request, but the rest could still be in ActiveRecord converting those results into Ruby objects.
Does your query for this request return a lot of rows? Are those rows very wide (like when you do a join of table1., table2., table3.*)?
I've had some experience in the past with serializers really really crushing performance. That usually ends up being a bit of a line by line hunt for what's responsible.
To troubleshoot this issue I recommend finding a way to get realtime or near realtime feedback on your performance. The newrelic_rpm gem has a /newrelic page you can view in development mode, which should provide feedback similar to Skylight. Skylight may have a similar development mode page you should look into.
There's also a gem called Peek that adds a little performance meter to each page view, that you can add gems to in order to show specific slices of the request, like DB, views, and even Garbage collection. https://github.com/peek/peek Check it out, especially the GC plugin.
Once you have that realtime feedback setup, and you can see something that maps roughly to your skylight output, you can start using a binary search in your code to isolate the performance problem.
In your controller, eliminate the rendering step by something like:
render json: {}
and look at the results of that request. If the performance improves dramatically then your issue is probably in the serialization phase.
If not, then maybe it is ActiveRecord blowing up the Ruby objectspace. Do a google search for Ruby Object Space profiling and you'll find ways to troubleshoot this.
If that's your problem, then try to narrow down the results returned by your query. select only the columns you need to render in this response. Try to eliminate joins if possible (by returning a foreign key instead of an object, if that is possible).
If serialization is your problem... Good luck. This one is particularly hard to troubleshoot in my experience. You may try using a more efficient JSON gem like OJ, or hardcoding your serializers rather than using ActiveRecord::Serializer (last resort!).
Good luck!
