EXISTS should normally return as soon as the subquery finds one row that satisfies its WHERE clause. So I think your query is as fast as you can make it.
I was a little surprised that LIMIT 1 seems to always speed up the query very slightly. I didn’t expect that. You can see the effect with EXPLAIN ANALYZE.
EXPLAIN ANALYZE
SELECT exists (SELECT 1 FROM table WHERE column = <value> LIMIT 1);