Quickly tested like this:
create table t(a jsonb);
insert into t select jsonb_object( array[(random() * 10)::int::text],
'{" "}') from generate_series(1, 100000);
insert into t select jsonb_object( array[(random() * 10)::int::text],
array[(random() * 1000)::int::text]) from generate_series(1, 100000);
explain analyze select * from t where a ? '1';
analyze t;
explain analyze select * from t where a ? '1';
Best, Alex