Concurrent connections - Server Sent Events with Spring Cloud Gateway - spring-boot

Question

A reactive Spring Boot application is used to generate Server Sent Events for clients subscribed by a key, e.g:
#GetMapping(path = "/sse", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<String> sse(#RequestParam String key) {
return Flux.interval(Duration.ofSeconds(2))
.map((i) -> key + " Fixed delay task - " + System.currentTimeMillis() / 1000);
}
Up to 10K concurrent Event Source connections can be established when run without Spring Cloud Gateway in front. With more connections the following exception is thrown:
2022-09-07 12:59:02.123 WARN 90517 --- [ctor-http-nio-1] io.netty.channel.DefaultChannelPipeline : An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception.
java.io.IOException: Too many open files
at java.base/sun.nio.ch.Net.accept(Native Method) ~[na:na]
at java.base/sun.nio.ch.ServerSocketChannelImpl.implAccept(ServerSocketChannelImpl.java:425) ~[na:na]
The problem occurs when Spring Cloud Gateway is in front of the service. The connection limit drops down to 2K connections. When exceeded, the service throws:
2022-09-07 13:09:37.590 WARN 90707 --- [ctor-http-nio-9] reactor.netty.channel.FluxReceive : [559c58ed-1, L:/127.0.0.1:9999 - R:/127.0.0.1:54567] An exception has been observed post termination, use DEBUG level to see the full stack: java.net.SocketException: Connection reset
And the gateway throws:
2022-09-07 12:59:55.696 ERROR 90264 --- [ctor-http-nio-2] a.w.r.e.AbstractErrorWebExceptionHandler : [7772ec3b-4438] 500 Server Error for HTTP GET "/sse?key=2682"
java.net.SocketException: Connection reset
at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394) ~[na:na]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Error has been observed at the following site(s):
*__checkpoint ⇢ org.springframework.cloud.gateway.filter.WeightCalculatorWebFilter [DefaultWebFilterChain]
*__checkpoint ⇢ HTTP GET "/sse?key=2682" [ExceptionHandlingWebHandler]
Original Stack Trace:
at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394) ~[na:na]
at java.base/sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:426) ~[na:na]
at io.netty.buffer.PooledByteBuf.setBytes(PooledByteBuf.java:258) ~[netty-buffer-4.1.79.Final.jar:4.1.79.Final]
The question is, why does Spring Cloud Gateway have such an impact on the number of concurrent connections?
The source code of the service app, the gateway, and a client JS app, used for receiving SSEs through multiple connections, is available on the git repo.