jsonb_agg performance

The jsonb_agg function seems to have significantly worse performance
than its json_agg counterpart:

=> explain analyze select pa.product_id, jsonb_agg(attributes) from
product_attributes2 pa group by pa.product_id;
                                                             
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
 GroupAggregate  (cost=1127.54..1231.62 rows=3046 width=380) (actual
time=28.632..241.647 rows=3046 loops=1)
   Group Key: product_id
   ->  Sort  (cost=1127.54..1149.54 rows=8800 width=380) (actual
time=28.526..32.826 rows=8800 loops=1)
         Sort Key: product_id
         Sort Method: external sort  Disk: 3360kB
         ->  Seq Scan on product_attributes2 pa 
(cost=0.00..551.00 rows=8800 width=380) (actual time=0.010..7.231
rows=8800 loops=1)
 Planning time: 0.376 ms
 Execution time: 242.963 ms
(8 rows)

=> explain analyze select pa.product_id, json_agg(attributes) from
product_attributes3 pa group by pa.product_id;
                                                             
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
 GroupAggregate  (cost=1136.54..1240.62 rows=3046 width=387) (actual
time=17.731..30.126 rows=3046 loops=1)
   Group Key: product_id
   ->  Sort  (cost=1136.54..1158.54 rows=8800 width=387) (actual
time=17.707..20.705 rows=8800 loops=1)
         Sort Key: product_id
         Sort Method: external sort  Disk: 3416kB
         ->  Seq Scan on product_attributes3 pa 
(cost=0.00..560.00 rows=8800 width=387) (actual time=0.006.5.568
rows=8800 loops=1)
 Planning time: 0.181 ms
 Execution time: 31.276 ms
(8 rows)

The only difference between the two tables is the type of the
attributes column (jsonb vs json).  Each table contains the same 8800
rows.  Even running json_agg on the jsonb column seems to be faster:

=> explain analyze select pa.product_id, json_agg(attributes) from
product_attributes2 pa group by pa.product_id;
                                                             
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
 GroupAggregate  (cost=1127.54..1231.62 rows=3046 width=380) (actual
time=30.626..62.943 rows=3046 loops=1)
   Group Key: product_id
   ->  Sort  (cost=1127.54..1149.54 rows=8800 width=380) (actual
time=30.590..34.157 rows=8800 loops=1)
         Sort Key: product_id
         Sort Method: external sort  Disk: 3360kB
         ->  Seq Scan on product_attributes2 pa 
(cost=0.00..551.00 rows=8800 width=380) (actual time=0.014..7.388
rows=8800 loops=1)
 Planning time: 0.142 ms
 Execution time: 64.504 ms
(8 rows)

Is it expected that jsonb_agg performance would be that much worse
than json_agg?