This is the "slow" plan (with inlining):

Nested Loop  (cost=1162.01..12106.68 rows=1 width=4) (actual time=75.084..164671.655 rows=5118 loops=1)
  Join Filter: (cte.id = public._test_pos.id)
  CTE cte
    ->  Hash Join  (cost=442.34..1162.01 rows=6667 width=4) (actual time=36.297..102.592 rows=20000 loops=1)
          Hash Cond: (public._test_pos.id = _test_time.id)
          ->  Seq Scan on _test_pos  (cost=0.00..428.00 rows=20000 width=4) (actual time=0.005..17.363 rows=20000 loops=1)
          ->  Hash  (cost=359.00..359.00 rows=6667 width=4) (actual time=36.274..36.274 rows=20000 loops=1)
                Buckets: 1024  Batches: 1  Memory Usage: 547kB
                ->  Seq Scan on _test_time  (cost=0.00..359.00 rows=6667 width=4) (actual time=0.008..18.628 rows=20000 loops=1)
                      Filter: ((some_time)::timestamp without time zone <= (('2012-05-15 00:00:00'::timestamp without time zone)::my_timestamp)::timestamp without time zone)
  ->  Seq Scan on _test_pos  (cost=0.00..10728.00 rows=1 width=4) (actual time=11.758..136.853 rows=5118 loops=1)
        Filter: ((('0101000020E61000000000000000001C400000000000001C40'::geography && _st_expand(pos, 300000::double precision)) AND (pos && '0101000020E61000000000000000001C400000000000001C40'::geography) AND _st_dwithin('0101000020E61000000000000000001C400000000000001C40'::geography, pos, 300000::double precision, true)) OR (('0101000020E61000000000000000001C400000000000001C40'::geography && _st_expand(pos, 400000::double precision)) AND (pos && '0101000020E61000000000000000001C400000000000001C40'::geography) AND _st_dwithin('0101000020E61000000000000000001C400000000000001C40'::geography, pos, 400000::double precision, true)))
  ->  CTE Scan on cte  (cost=0.00..133.34 rows=6667 width=4) (actual time=0.008..16.017 rows=20000 loops=5118)
Total runtime: 164678.566 ms


This is the "fast" plan (without inlining):

Hash Join  (cost=23401.07..39268.53 rows=3334 width=4) (actual time=158.753..271.443 rows=5118 loops=1)
  Hash Cond: (cte.id = public._test_pos.id)
  CTE cte
    ->  Merge Join  (cost=2639.22..12673.05 rows=666700 width=4) (actual time=72.092..155.530 rows=20000 loops=1)
          Merge Cond: (_test_time.id = public._test_pos.id)
          ->  Sort  (cost=782.45..799.12 rows=6667 width=4) (actual time=36.718..52.012 rows=20000 loops=1)
                Sort Key: _test_time.id
                Sort Method: quicksort  Memory: 1294kB
                ->  Seq Scan on _test_time  (cost=0.00..359.00 rows=6667 width=4) (actual time=0.013..18.769 rows=20000 loops=1)
                      Filter: ((some_time)::timestamp without time zone <= (('2012-05-15 00:00:00'::timestamp without time zone)::my_timestamp)::timestamp without time zone)
          ->  Sort  (cost=1856.77..1906.77 rows=20000 width=4) (actual time=35.365..50.692 rows=20000 loops=1)
                Sort Key: public._test_pos.id
                Sort Method: quicksort  Memory: 1294kB
                ->  Seq Scan on _test_pos  (cost=0.00..428.00 rows=20000 width=4) (actual time=0.006..16.802 rows=20000 loops=1)
  ->  CTE Scan on cte  (cost=0.00..13334.00 rows=666700 width=4) (actual time=72.097..190.022 rows=20000 loops=1)
  ->  Hash  (cost=10728.00..10728.00 rows=1 width=4) (actual time=56.369..56.369 rows=5118 loops=1)
        Buckets: 1024  Batches: 1  Memory Usage: 140kB
        ->  Seq Scan on _test_pos  (cost=0.00..10728.00 rows=1 width=4) (actual time=11.355..50.909 rows=5118 loops=1)
              Filter: ((('0101000020E61000000000000000001C400000000000001C40'::geography && _st_expand(pos, 300000::double precision)) AND (pos && '0101000020E61000000000000000001C400000000000001C40'::geography) AND _st_dwithin('0101000020E61000000000000000001C400000000000001C40'::geography, pos, 300000::double precision, true)) OR (('0101000020E61000000000000000001C400000000000001C40'::geography && _st_expand(pos, 400000::double precision)) AND (pos && '0101000020E61000000000000000001C400000000000001C40'::geography) AND _st_dwithin('0101000020E61000000000000000001C400000000000001C40'::geography, pos, 400000::double precision, true)))
Total runtime: 276.209 ms