Why Nginx 0 second latency and 1 second latency have much different ? And how to calculate it?

I executed 1-second latency load test using jmter. Load test directly executed for Nginx mock service.

Load testing jmeter details: Users: 250 Ramp up (seconds): 125 Duration: 36000 (10 hrs)

Mock backend details: Nginx server using as mock service. 1 sec latency added (echo_sleep 1)

Results 249.4/s tps

But I executed the same script when backend latency is 0 seconds. Results 276939.2/s tps

1. Why the tps so much drop when the backend is having 1-second latency? (I used default configuration of the nginx)
2. How can I calculate the expected tps for n second latency for above jmeter script (user count, Ramp up, Duration, Backend latency)?