Firstly the sensor value is read into the 'reading' variable. This could take up to a few hundred microseconds.
Except for the first reading, about 100 microseconds. (13 cycles of a 125kHz clock)
Not "a few hundred microseconds".
The first will take about 200 microseconds. (25 cycles of a 125kHz clock)
Next, the 'reading' variable is then sent via Serial comms to the computer. Depending on the baud rate, this could take up to a few milliseconds to complete.
Since about Arduino 1.0 the output is buffered, and will be sent out during the "delay()", so the completion time will be during the delay.
// Code in here gets executed every time the counter
// reaches, in this case, 6250 (every 100 ms)
reading = analogRead(sensorPin);
IMO, an ISR is not the place to be doing an analogRead. Too slow.
Initiate a read in an ISR and interrupt again on completion, but don't sit in an ISR doing a busy-wait.