Question: I have been tinkering around Flask and FastAPI to see how it acts as a server. One of the main things that I would like to know is how Flask and FastAPI deal with multiple requests from multiple clients. Especially when the code has efficiency issues (long database query time). So, I tried making a simple code to understand this problem. The code is simple, when the client access the route, the application sleeps for 10 seconds before it returns results. It looks something like this: FastAPI import uvicorn from fastapi import FastAPI from time import sleep app = FastAPI() @app.get('/') async def root(): print('Sleeping for 10') sleep(10) print('Awake') return {'message': 'hello'} if __name__ == "__main__": uvicorn.run(app, host="127.0.0.1", port=8000)  Flask from flask import Flask from flask_restful import Resource, Api from time import sleep app = Flask(__name__) api = Api(app) class Root(Resource): def get(self): print('Sleeping for 10') sleep(10) print('Awake') return {'message': 'hello'} api.add_resource(Root, '/') if __name__ == "__main__": app.run()  Once the applications are up, I tried accessing them at the same time through 2 different chrome clients. The below are the results: FastAPI  Flask  As you can see, for FastAPI, the code first waits 10 seconds before processing the next request. Whereas for Flask, the code processes the next request while the 10-second sleep is still happening. Despite doing a bit of googling, there is not really a straight answer on this topic. If anyone has any comments that can shed some light on this, please drop them in the comments. Your opinions are all appreciated. Thank you all very much for your time. EDIT An update on this, I am exploring a bit more and found this concept of Process manager. For example, we can run uvicorn using a process manager (gunicorn). By adding more workers, I am able to achieve something like Flask. Still testing the limits of this, however. https://www.uvicorn.org/deployment/ Thanks to everyone who left comments! Appreciate it.

Answer: This seemed a little interesting, so i ran a little tests with ApacheBench: Flask from flask import Flask from flask_restful import Resource, Api app = Flask(__name__) api = Api(app) class Root(Resource): def get(self): return {"message": "hello"} api.add_resource(Root, "/")  FastAPI from fastapi import FastAPI app = FastAPI(debug=False) @app.get("/") async def root(): return {"message": "hello"}  I ran 2 tests for FastAPI, there was a huge difference:  gunicorn -w 4 -k uvicorn.workers.UvicornWorker fast_api:app uvicorn fast_api:app --reload  So here is the benchmarking results for 5000 requests with a concurrency of 500: FastAPI with Uvicorn Workers Concurrency Level: 500 Time taken for tests: 0.577 seconds Complete requests: 5000 Failed requests: 0 Total transferred: 720000 bytes HTML transferred: 95000 bytes Requests per second: 8665.48 [#/sec] (mean) Time per request: 57.700 [ms] (mean) Time per request: 0.115 [ms] (mean, across all concurrent requests) Transfer rate: 1218.58 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 6 4.5 6 30 Processing: 6 49 21.7 45 126 Waiting: 1 42 19.0 39 124 Total: 12 56 21.8 53 127 Percentage of the requests served within a certain time (ms) 50% 53 66% 64 75% 69 80% 73 90% 81 95% 98 98% 112 99% 116 100% 127 (longest request)  FastAPI - Pure Uvicorn Concurrency Level: 500 Time taken for tests: 1.562 seconds Complete requests: 5000 Failed requests: 0 Total transferred: 720000 bytes HTML transferred: 95000 bytes Requests per second: 3200.62 [#/sec] (mean) Time per request: 156.220 [ms] (mean) Time per request: 0.312 [ms] (mean, across all concurrent requests) Transfer rate: 450.09 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 8 4.8 7 24 Processing: 26 144 13.1 143 195 Waiting: 2 132 13.1 130 181 Total: 26 152 12.6 150 203 Percentage of the requests served within a certain time (ms) 50% 150 66% 155 75% 158 80% 160 90% 166 95% 171 98% 195 99% 199 100% 203 (longest request)  For Flask: Concurrency Level: 500 Time taken for tests: 27.827 seconds Complete requests: 5000 Failed requests: 0 Total transferred: 830000 bytes HTML transferred: 105000 bytes Requests per second: 179.68 [#/sec] (mean) Time per request: 2782.653 [ms] (mean) Time per request: 5.565 [ms] (mean, across all concurrent requests) Transfer rate: 29.13 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 87 293.2 0 3047 Processing: 14 1140 4131.5 136 26794 Waiting: 1 1140 4131.5 135 26794 Total: 14 1227 4359.9 136 27819 Percentage of the requests served within a certain time (ms) 50% 136 66% 148 75% 179 80% 198 90% 295 95% 7839 98% 14518 99% 27765 100% 27819 (longest request)  Total results Flask: Time taken for tests: 27.827 seconds FastAPI - Uvicorn: Time taken for tests: 1.562 seconds FastAPI - Uvicorn Workers: Time taken for tests: 0.577 seconds  With Uvicorn Workers FastAPI is nearly 48x faster than Flask, which is very understandable. ASGI vs WSGI, so i ran with 1 concurreny: FastAPI - UvicornWorkers: Time taken for tests: 1.615 seconds FastAPI - Pure Uvicorn: Time taken for tests: 2.681 seconds Flask: Time taken for tests: 5.541 seconds I ran more tests to test out Flask with a production server. 5000 Request 1000 Concurrency Flask with Waitress Server Software: waitress Server Hostname: 127.0.0.1 Server Port: 8000 Document Path: / Document Length: 21 bytes Concurrency Level: 1000 Time taken for tests: 3.403 seconds Complete requests: 5000 Failed requests: 0 Total transferred: 830000 bytes HTML transferred: 105000 bytes Requests per second: 1469.47 [#/sec] (mean) Time per request: 680.516 [ms] (mean) Time per request: 0.681 [ms] (mean, across all concurrent requests) Transfer rate: 238.22 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 4 8.6 0 30 Processing: 31 607 156.3 659 754 Waiting: 1 607 156.3 658 753 Total: 31 611 148.4 660 754 Percentage of the requests served within a certain time (ms) 50% 660 66% 678 75% 685 80% 691 90% 702 95% 728 98% 743 99% 750 100% 754 (longest request)  Gunicorn with Uvicorn Workers Server Software: uvicorn Server Hostname: 127.0.0.1 Server Port: 8000 Document Path: / Document Length: 19 bytes Concurrency Level: 1000 Time taken for tests: 0.634 seconds Complete requests: 5000 Failed requests: 0 Total transferred: 720000 bytes HTML transferred: 95000 bytes Requests per second: 7891.28 [#/sec] (mean) Time per request: 126.722 [ms] (mean) Time per request: 0.127 [ms] (mean, across all concurrent requests) Transfer rate: 1109.71 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 28 13.8 30 62 Processing: 18 89 35.6 86 203 Waiting: 1 75 33.3 70 171 Total: 20 118 34.4 116 243 Percentage of the requests served within a certain time (ms) 50% 116 66% 126 75% 133 80% 137 90% 161 95% 189 98% 217 99% 230 100% 243 (longest request)  Pure Uvicorn, but this time 4 workers uvicorn fastapi:app --workers 4 Server Software: uvicorn Server Hostname: 127.0.0.1 Server Port: 8000 Document Path: / Document Length: 19 bytes Concurrency Level: 1000 Time taken for tests: 1.147 seconds Complete requests: 5000 Failed requests: 0 Total transferred: 720000 bytes HTML transferred: 95000 bytes Requests per second: 4359.68 [#/sec] (mean) Time per request: 229.375 [ms] (mean) Time per request: 0.229 [ms] (mean, across all concurrent requests) Transfer rate: 613.08 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 0 20 16.3 17 70 Processing: 17 190 96.8 171 501 Waiting: 3 173 93.0 151 448 Total: 51 210 96.4 184 533 Percentage of the requests served within a certain time (ms) 50% 184 66% 209 75% 241 80% 260 90% 324 95% 476 98% 504 99% 514 100% 533 (longest request)