When you open a filter in Looker (i.e. on a string field), Looker runs a simple
SELECT DISTINCT <field name> FROM <table> LIMIT 1000 query to get the first 1000 distinct options as suggestions.
If that first query returns before the user starts / stops typing in the filter box, and there are fewer than 1000 results, Looker will make subsequent suggestions very quickly because it knows the universe of possibilities for that field.
If some other text is written in the filter box (e.g. “goodlooker”) and the user stops typing (for 300ms), any previous running suggestion query will get killed and a new one will run with a filter on it:
WHERE <field> ilike 'goodlooker%'
If this is a concern because you are suggesting on large fact tables, or don’t want extra queries running on your database, you have a few options: