Will It Work on an Old or Slow Laptop?
Yes. Reading and writing metadata is light work. Conversions are the only heavy part.
Writing a date into a photo barely touches the processor. An old laptop does it at much the same speed as a new one, because the disk is the limit in both cases.
Converting HEIC to JPG, or videos to MP4, is where an older machine will feel slower. You can leave those switched off and do the dates now, then convert later if you ever need to.
Takeout JSON Metadata Fixer lets you turn each of those on separately, so you only pay for the work you actually want. It runs on your own computer and is free for your first 500 photos.