Do you know what "X" is? or have the time/patience to get close enough? If so, would it be unreasonable to have your script perform only a few at a time? You could catch the time at the beginning of the loop through the domains, and if it is going to fast, just pause the program for a while with a syscall. If you can't have the program running all day, just run it at night and keep track of which domains you have done already.