Why is my query taking so long if my field is correctly indexed?
If you have a 5400 RPM hard drive, this is almost exactly how much time you would expect it to take to read 8942 blocks if there was no effective read-ahead or read re-ordering (as implemented by the kernel, with some collaboration from PostgreSQL).  You can confirm if the time goes to disk reads by turning track_io_timing on before doing the EXPLAIN (ANALYZE, BUFFERS).

Even with a single 5400 hdd, I've seen it get increased performance, up to 2x, by cranking up effective_io_concurrency to a large value, like 50 or 100.

If you repeat the query, is it faster the second time?  Is your goal to make this work from a hot-cache or a cold-cache?  How much RAM do you have?