Return the first number found greater than the provided input number (13 digits)
I'm having issues with it being very slow. 


What makes your script code slow is that you're reading in the file yourself with the while loop, and apply grep to each single input line, instead of passing grep the file itself and let it just do it's job.

No matter what you want to search for with grep, you should always first pass your input to it with a single call, and inspect the results afterwards.


  For the output I wanted the line number in the file (for slicing) as well as the original line (to confirm/inspect).


grep already has this feature intrinsically (at least as it says from this documentation):


  
    -n, --line-number
           Prefix each line of output with the line number within its input file.
  


you simply can do what you want using this option.

Thus you can get rid of your while loop and count variable to determine the line number yourself.