I saw something similar recently when reading JSON data from postgres via Rails, which was unbelievably slow. I found the reason to be that Rails would automatically parse the JSON value into a in-memory ruby object. Since this was for a API-like backend, the solution was simple: just read the column as a string ("SELECT payload::varchar..")

I don't know if dkango is doing the same, but using an ORM might just do that to you.