Keyboard Maestro runs actions at 1000 actions per second. So to scan a folder with 10,000 items using a For Each action will take at a minimum 10 seconds per action within the For Each action.
Keyboard Maestro is not designed for high speed use, if you want to process a large number of files, use a script. If you want to scan a folder and find a specific set of subfiles, perhaps use a script that uses the find command. Similarly for lines, use the grep command.
Once you have whittled down the list to a manageable number, then use Keyboard Maestro actions to process them.