Slow select

I have a table with column of character varying(100). There are about
150.000.000 rows in a table. Index was created as

CREATE INDEX idx_stringv
  ON bn_stringvalue
  USING btree
  (lower(value::text));

I'm trying to execute queries like 'select * from stringvalue where
value=lower(?)'. Making 1000 selects takes about 4-5 min. I did vacuum and
analyze on this table and checked that query plan uses index. What can I do
to make it faster?

Thanks in advance, Yulia
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