Hey primoz-2, welcome to the ROG forum!
The 1600GHz is the idle speed of your CPU - start some benchmark/load your CPU, then it will go higher. It's a built in mechanism to reduce the voltage on the CPU while it's not needed (it will go back as soon as you start loading it).
You can simply have your CPU at permanent 3.5GHZ by enabling "HIgh Performance" in the Windows Power Saving Plans (Right now perhaps is in "Balanced Mode").