Spotlight is invaluable for internal drives, I use it all the time for finding items and even launching apps. If you are a quick typist it’s much faster than clicking. But if you have an external drive that you swap from machine to machine (for example: for making backups) then bringing the drive to a new machine will invoke Spotlight indexing, which instantly slows down any operation you might want to prosecute with said drive.
Fortunately, there is a quick way to disable it for a given volume. Open the Terminal and enter:
sudo mdutil -i off //Volumes/your_volume_name
You can escape any spaces in the volume name by encasing everything in quotes, or by escaping spaces (forcing them) with a “\”
sudo mdutil -i off //Volumes/My\ volume\ name