I had the same problem. After doing some research i realized that the hub has be to able to transmit HDMI 2.0 signals, which allows for 60hz refresh rates. If it isnt able to do HDMI 2.0 then the picture maxes out at 30hz refresh rate. Most USB C hubs on the market currently are not compatable with HDMI 2.0, so you need to find one which specifically says it does support 2.0.
Another fix to the problem is to get a USB C to DisplayPort cable. Displayport cables work natively at 60hz. This is how i fixed the problem on my monitor and it now works perfectly smooth.
Hope this helps!