How big does the data stored in that field get? More than 2KB? Real
question- is it getting stored plain, compressed inline, or toasted? Have
you set the storage strategy/type, or is it the "extended" default behavior
that compresses and then stores in the toast table if still more than 2000
bytes?
Always joining on a field that is toasted and compressed is going to be
considerably slower than a discrete field. Do you have an index (or
several) on the json field values? Also, are we actually talking about
jsonb?