Slow INNER JOIN planning time

Master database

localhost [classroom_prod] # EXPLAIN ANALYZE SELECT * FROM knowledge_snapshots ks INNER JOIN answers a ON a.knowledge_snapshot_id = ks.id WHERE ks.student_id = 7538860;
                                                                                      QUERY PLAN                                                                        
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.87..2899.01 rows=235 width=105) (actual time=0.095..5.197 rows=72 loops=1)
   ->  Index Scan using knowledge_snapshots_parent_student_id_created_at_idx on knowledge_snapshots ks  (cost=0.43..95.05 rows=26 width=53) (actual time=0.047..0.086 rows=6 loops=1)
         Index Cond: (student_id = 7538860)
   ->  Index Scan using answers_parent_knowledge_snapshot_id_idx on answers a  (cost=0.44..106.56 rows=128 width=52) (actual time=0.838..0.845 rows=12 loops=6)
         Index Cond: (knowledge_snapshot_id = ks.id)
 Planning time: 15.655 ms
 Execution time: 5.288 ms
(7 rows)

Standby database

db-standby-1-prod [classroom_prod] # EXPLAIN ANALYZE SELECT * FROM knowledge_snapshots ks INNER JOIN answers a ON a.knowledge_snapshot_id = ks.id WHERE ks.student_id = 7538860;
                                                                                      QUERY PLAN                                                                                      
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.87..2899.01 rows=236 width=105) (actual time=0.080..0.301 rows=72 loops=1)
   ->  Index Scan using knowledge_snapshots_parent_student_id_created_at_idx on knowledge_snapshots ks  (cost=0.43..95.05 rows=26 width=53) (actual time=0.028..0.063 rows=6 loops=1)
         Index Cond: (student_id = 7538860)
   ->  Index Scan using answers_parent_knowledge_snapshot_id_idx on answers a  (cost=0.44..106.56 rows=128 width=52) (actual time=0.022..0.028 rows=12 loops=6)
         Index Cond: (knowledge_snapshot_id = ks.id)
 Planning time: 169.347 ms
 Execution time: 0.404 ms
(7 rows)

Table definitions

db-standby-1-prod [classroom_prod] # \d+ answers
                                                             Table "public.answers"
        Column         |           Type           |                      Modifiers                       | Storage  | Stats target | Description 
-----------------------+--------------------------+------------------------------------------------------+----------+--------------+-------------
 math_question_id      | character varying(12)    | not null                                             | extended |              | 
 student_id            | integer                  | not null                                             | plain    |              | 
 created_at            | timestamp with time zone | not null default now()                               | plain    |              | 
 id                    | bigint                   | not null default nextval('answers_id_seq'::regclass) | plain    |              | 
 duration_seconds      | smallint                 |                                                      | plain    |              | 
 knowledge_snapshot_id | integer                  | not null                                             | plain    |              | 
 correctness           | real                     | not null                                             | plain    |              | 
 skill_id              | character varying(8)     | not null                                             | extended |              | 
Indexes:
    "answers_parent_pkey" PRIMARY KEY, btree (id)
    "answers_parent_knowledge_snapshot_id_idx" btree (knowledge_snapshot_id)
    "answers_parent_student_id_created_at_idx" btree (student_id, created_at DESC)
Foreign-key constraints:
    "answers_knowledge_snapshot_id_fkey" FOREIGN KEY (knowledge_snapshot_id) REFERENCES knowledge_snapshots(id)
    "answers_math_question_id_fkey" FOREIGN KEY (math_question_id) REFERENCES math_questions(id)
    "answers_student_id_fkey" FOREIGN KEY (student_id) REFERENCES students(id)

db-standby-1-prod [classroom_prod] # \d+ knowledge_snapshots
                                                             Table "public.knowledge_snapshots"
        Column         |           Type           |                            Modifiers                             | Storage  | Stats target | Description 
-----------------------+--------------------------+------------------------------------------------------------------+----------+--------------+-------------
 id                    | bigint                   | not null default nextval('knowledge_snapshots_id_seq'::regclass) | plain    |              | 
 student_id            | integer                  | not null                                                         | plain    |              | 
 current_standard      | text                     | not null                                                         | extended |              | 
 sub_standard_perc     | real                     | not null                                                         | plain    |              | 
 sub_sub_standard_perc | real                     |                                                                  | plain    |              | 
 created_at            | timestamp with time zone | not null default now()                                           | plain    |              | 
 domain_id             | character varying(8)     | not null                                                         | extended |              | 
 standard_id           | character varying(8)     | not null                                                         | extended |              | 
Indexes:
    "knowledge_snapshots_parent_pkey" PRIMARY KEY, btree (id)
    "knowledge_snapshots_parent_student_id_created_at_idx" btree (student_id, created_at DESC)
Foreign-key constraints:
    "knowledge_snapshots_domain_id_fkey" FOREIGN KEY (domain_id) REFERENCES domains(id)
    "knowledge_snapshots_standard_id_fkey" FOREIGN KEY (standard_id) REFERENCES standards(id)
Referenced by:
    TABLE "answers" CONSTRAINT "answers_knowledge_snapshot_id_fkey" FOREIGN KEY (knowledge_snapshot_id) REFERENCES knowledge_snapshots(id)
Triggers:
    populate_knowledge_snapshots_standard_id_column BEFORE INSERT ON knowledge_snapshots FOR EACH ROW WHEN (new.standard_id IS NULL) EXECUTE PROCEDURE populate_knowledge_snapshots_standard_id_column()