Deterioration in performance when query executed in multi threads

Hi all,
We are running a stress test that executes one select query with multiple threads.
The query executes very fast (10ms). It returns 100 rows. I see deterioration in the performance when we have multiple threads executing the query. With 100 threads, the query takes between 3s and 8s.

I suppose there is a way to tune our database. What are the parameters I should look into? (shared_buffer?, wal_buffer?)

Thanks for your help,
Anne