How to increase ADC sample rate (Arduino MEGA 2560-16U)

When I use analogRead(A9) in my loop(), it seems my sample rate is only 4kHz.
In my sketch, I compare the ADC value to midscale (512), and if so, write a HIGH, then immediately write a LOW.

void loop()
{
ADC_v = analogRead(A9);
ADC_i = analogRead(A7);
if (ADC_v > 512)
{digitalWrite(13, HIGH);}
{digitalWrite(13, LOW);}

if (ADC_i > 102)
{digitalWrite(5, HIGH);}
{digitalWrite(5,LOW);}
}

The scope capture shows samples are spaced at 250usec (4kHz). Is there a way to make this faster?

If you want faster ADC performance, you can set up a timer to trigger the ADC which generates an interrupt when the conversion is complete. In the ADC interrupt service routine you handle the sample.