I need to know if I can rely on Qt socket solution to make a well optimized game server that can handle at least 50.000 Players in the future? I mean is Qt socket fast enough?
Or is there an other fast solution (multi-platform if possible: Windows, Linux..) for C++ ??
Qt sockets will base on the system sockets. So they certainly will be fast enough. I don't think that using other types of sockets will be considerable faster or slower.
You might as well consider not using sockets at all, and try asynchronous, non-blocking strategy (epoll / select). If your game has to scale to 50000 simultaneous connections, standard thread-per-connection approach will probably kill your system's scheduler, or get you out of virtual memory (50000 connections would consume about 50 GB of RAM, if typical 1MB large stacks are used).