When I try to browse user X, it takes a long time (30 seconds – 1 minute) before the tab shows. Why?
Most likely, user X is behind a firewall that silently drops incoming connections. You have to wait a long time before the connection times out and then ask the server to notify user X’s client that you want to connect to him/her. This timeout can be avoided by enabling the “I can receive direct connections” in the Settings window (in the “Server” section), in which case the nicotine client will skip the process of trying to connect to the other end, and just ask the server to ask user X to connect to you.