I am not using darknet but Matrix1 Utils Sockets, I have been able to let work the UDP packets and I have converted my code to use these ones instead of the TCP ones for passing data to and from the server, while using TCP only for logging in, get servers list, etc.
With UDP packets everything is much smoother and I don't have the choppy movements anymore, probably they still put down the fps but much less, as if I open the game with almost low settings except water with medium settings, I get almost 50 fps, and if I open 2 istances of the game on the same pc I get almost the same fps on both of them, so the network is limiting the fps, but as I use to cap the framerate at 50 fps it's fine.
5 updates per seconds could be few compared with other games that as I have seen use 20-30 updates per second, but for my type of game are fine (I have already implemented the interpolation part and everything is smooth now) and this allow me to use cheap servers and don't overload them, as I still need to manage 32 players per server.