I saw a thing on my GDS-1052-U oscilloscope: if I set the time base to 25mS/div there appear a delay of the signal on the screen. So, when I rotate the knob for the time base to 25mS/div, there is a delay until the trace appear on the screen. If I set the time base to 5mS/div there is almost no delay. If I set the knob for 10mS/div the delay is smaller than the delay for 25mS/div. This happens on both channels. The probe is not connected to any signal source while I do these things. The voltage/division is 20mV/div. Is that normal ?