Degraded SQL Query Speed By Nesting a Single Query inline vs temp table - postgresql

Question

I have a query of the following, basic form.
SELECT DISTINCT
a.field1,
b.field2,
c.agg_values
FROM a
INNER JOIN b ON a.something = b.something
LEFT JOIN (
SELECT
array_to_string(array_agg(label), ';;') AS agg_values,
some_table.some_field
FROM some_table
WHERE some_table.some_field = 'some-fixed-value'
GROUP BY some_field
) AS c ON a.some_field = c.some_field
WHERE a.some_other_field = 'some-other-fixed-value'
There's nothing too wild about this query. Pretty run of the mill!
This query runs pretty slow in my Postgres 9.4.5 (~4 minutes), where I have maybe 15k records returned total. some_table has probably ~10k records.
If I move the content of that LEFT JOIN sub-query to a temp table and left join from the temp table, my performance increases substantially. My query may take only 15s now, vs 240s. To be more explicit, if I remove SELECT array_to_string ... GROUP BY some_field query, and put that query into a temp table, then left join on that temp table, BAM, fast.
CREATE TEMP TABLE temp_table_c ( ... );
INSERT INTO temp_table_c SELECT ... same query nested in LEFT JOIN from before ...;
SELECT DISTINCT
a.field1,
b.field2,
c.agg_values
FROM a
INNER JOIN ON a.something = b.something
LEFT JOIN temp_table_c AS c ON a.some_field = c.some_field
WHERE a.some_other_field = 'some-other-fixed-value'
I would appreciate it if someone could explain why the TEMP TABLE version of the query is so much more performant.
Thanks!