$ mvn spring-boot:run
# (in another terminal)
$ mvn gatling:test
$ open target/gatling/performancesimulation-*/index.html

The endpoint /io simulates IO by sleeping for an interval between 200 and 700ms. If you see requests taking longer than 700ms to fulfil, then there's a bottleneck somewhere.

1. Check ulimit -n, the number of file descriptiors your process can have open at a time. Number of file descriptors == number of connections, for both the server and the performance test.
2. Change server.tomcat.max-threads in application.properties. By default Spring Boot uses an embedded Tomcat server, and the default maximum threads if not set is 200.