Elasticsearch async search very slow

i'm running queries on a 3 node cluster. Simple query_string query with some wildcards. When i use _search, it takes about 10-15 ms to execute the query but when using _async_search query time goes up to around 300 000 ms. Can anybody help, what could be wrong? The queries are exactly the same, Elasticsearch 7.17.5 has 8 GB heap set, 2 vcores, nothing else is running on those machines. The data set is on 1 shard and occupies ~160 GB (this is a test env.) No matter if i search in one index or multiple indexes, it's very_veeery_veeeeery slow. Maybe a "hidden" setting should be adjusted to get acceptable performance? Thank you!