macbook pro external monitor lag/safari 99% cpu

Hi there,

I'm trying to using my mbp 13" 2016 with an external monitor (Benq pd3200u) connected with a usb-c --> DisplayPort cable and a Magic Mouse + Magic keyboard and I'm issuing some problems:

• the mouse cursor lag: trying to moving the mouse every 5-10sec I'm issuing a freeze of the pointer
• safari is consuming 99% of cpu when I move the cursor above a YouTube video (low resolution: 240/360)

some details:

• everything works good when I use the laptop standalone without any external device
• flash is not installed
• i've tryed to change the monitor resolution but doesn't solve the problem
• I'm not using any dock station/adapter
• i've tryed to swap the usb-c port (my mbp have 4 usb-c port) but doesn't solve the problem with safari while mouse movement seems ok except that when I click to pause e video often it double click (so pause and play the video)
• 100MB internet connection with orbi wi-fi system 2-3meters far from the mbp

anyone issued my problems?

thanx

Denis