Help request to improve function performance

Dear members of the list,
I have a function which returns a custom type, that has only two fields,
each of them being varchar arrays.
The reason that I have written this function is that I have a table
basically with the following structure (with simplified column names)

name_col1 name_col2 sessionId
value1 value3 id1
value2 value2 id1
value4 value4 id1
value7 value4 id2
value2 value2 id2
value4 value4 id2
value1 value5 id3

So mutliple rows are bound together with sessionIds, and I need to get back
all rows with a query, grouped by sessionID. However, group by sql statement
does not solve my problem, since I get back a lot of rows, which I have to
group into objects again in my application. What I need is a way to return
all rows having the same sessionId as a single row. Of course this is not
possible with this table, so I've created a custom type, which has array
type columns. The following function gets all rows that belongs to a
patient, and for each session id, it inserts rows with that session id into
array fields of the custom type.
The problem is, it is very slow! Getting back all the rows with a select
takes 360 ms, while getting back the results of this function takes 50
seconds! Is there any way I can make the following function faster, or any
other methods you can recommend to do what I'm trying to do? I am trying to
avoid hundreds of calls to db, or grouping query results in my middleware
application. Here comes the function, and your help will be much
appreciated.

Best Regards
Seref

CREATE or REPLACE FUNCTION getNodeContainers( context_Id varchar) RETURNS
setof NodesContainer AS
$$
DECLARE
archetype_data_row app.archetype_data%ROWTYPE;
archetype_data_row_main app.archetype_data%ROWTYPE;
nodescontainervar NodesContainer%ROWTYPE;
session_Id varchar;
indexVar integer := 0;
BEGIN
CREATE TEMP TABLE all_rows_of_patient AS select * from
app.archetype_data
WHERE app.archetype_data.context_id = context_Id;
FOR session_Id IN
SELECT distinct(all_rows_of_patient.session_id) from
all_rows_of_patient
LOOP -- do the following for each session_ID
indexVar := 0;
FOR archetype_data_row IN --select rows that belong to this session ID
SELECT * from all_rows_of_patient
WHERE all_rows_of_patient.session_id = session_Id and
all_rows_of_patient.context_id = context_Id
LOOP
nodescontainervar.name[indexVar] := archetype_data_row.name;
nodescontainervar.context_Id[indexVar] := archetype_data_row.context_Id;
indexVar := indexVar + 1;
END LOOP;
return NEXT nodescontainervar;
END LOOP;
drop table all_rows_of_patient;
return;
END;
$$ LANGUAGE 'plpgsql';

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