It seems that only the USB Hub pictured below contain a special chinese chip (some fe1.1 type ) that allow webcam streaming in full resolution with Linux.
With this hub , Mjpgstreamer and uvcstreamer work well in MJPEG (10 fps at 640x480 with my cheap and extra small webcam).
With other hubs, I have lower performances and can only stream in YUV.
For space reason I am going to replace this hub with a common USB 2.0 hub, with a smaller pcb size 3 x 2 cm. I will still be able to stream in YUV at 5 fps which is enough for me now.