Lambda billing duration for normal traffic

While I made single request at a time, lambda duration is 500 ms or less. But when I set 50 users concurrency and 10 users started per sec (spawn rate) in LOCUST, lambda duration would be 15000 ms. My Mongodb free tier is 500 connection limit and limit is not reached anytime. When I give traffic , billing duration would be more but 500 ms for very small traffic. Is it lambda side problem or Mongodb side problem?

**I have made load testing on single api.