pg_stat_activity - is it possible to decompose a stored procedure further?
No, but given the function is read-only, one can try running it again in the concurrent session and see if the slowdown is reproducible. 

If it is, try running the same queries as this function does manually. You may find them by running \sf+ on the function in psql, or, if you are running Bucardo5, by looking at the schema definition: https://github.com/bucardo/bucardo/blob/master/bucardo.schema