Understanding how Visa reads device buffers

Hi
I have a device Keithley K2602B Source meter which supports NI VISA based "GPIB" communication

Let us say that the I measured four current values and saved these measurements in Buffer1. Now device buffer1 has 4 readings in it

Say I have something like :

for ( i =1; i<=4; i++)

{

 str =  ReadString(Int32count);  // Reads a string from the formatted read buffer. This is taken from VISA Implementation
Specification for .NET Documentation by NI VISA

}

When  i = 1, the data is received from the Device buffer1 in 200 milliseconds.

When i = 2,3,4 the data is received from device buffer1 in 1 milliseconds respectively

Why does it take a longer time to get the 1st data from the device while the other retrievals take less time ??

Please share this info