Filter dropdown is slow

6 millions rows.... takes approx 20 seconds to load the dropdown !!
how can i make it fast..??

for that column i have already tried making indexes and Text Search Indexes: using gin in production DB. which helps after i select and click add to filter.


Queries are already faster. I want to improve the filter dropdown.
unique values of more than 300. so a search box is coming. so when typing it takes a huge 20 seconds to lead the result.
I want to make that fast.

I have similar problem but linked filters