It may impact total speed of USB connection since there are two more connection points for cable. Those points will introduce noise in connection and some of data will need to be retransmitted.
However, USB is quite resilient and while you may see some drop in speed (and occasional latency increase), it will be barely detectable. You would need device that uses USB to it's maximum in order to notice that on 50cm extension cable.
Keystrokes, mouse clicks and even hard drives will not feel significant drop in performance.
Thing that may give you more worries is voltage drop across that length. Some power-hungry devices (hard drives) may not get enough juice if total wire length gets too long. Of course, this also depends on cable quality.