Thanks, robtillaart. I'm using the Arduino 1.5.1r2. Alack, there are no such functions as analogReadStart() and analogReadReady().
I agree with ralphnev, that Atmel Studio should be used to optimize code on a low level. It remains only to find out how to combine the Arduino and Atmel Studio. 
Is it possible to add into Arduino language the functions from Atmel Studio? Give me links to documents on this issue, please.