Webrick is a single-thread, single-process web server.
Webrick will keep the router's connection open until it has downloaded the entirety of the request from the router. The router then will move on to the next request. Webrick server will then take the request, run the application code and send back the response to the router.
During the all of time, your host is busy, and will not accept connections from other routers. If the router attemps to connect to this host while the request is being processed, the router will wait until the host is ready.
The problem with Webrick are exaggerated with slow requests and uploads. If someone is trying to upload a big file, Webrick is going to sit there and wait while the request downloads, and will not download anything in the meantime, and it also will not accept any requests.