Yes, either type G when presented with the top of the file or pass +G on the command line. It will then read all the intervening file to calculate the line numbers, and tells you it's doing this and to interrupt it to stop. To prevent it doing this in the first place use the -n option.

"less -n huge_file" and then G only reads the start and end of the file. Type G again to move to the latest end of the file if it's growing.

Yes. It is CPU intensive (see auntie post above).