You can try vinegar, CRL or Lime Away but if the rim holes are clogged I take a coathanger and clear them one by one.
If the rim holes are loaded up with calcium what does the jet look like? Does the flush go down on the first flush? Back to you, n Tom

Jet is OK, water is just starting to go slow through the holes don't want it to get any worse. I will take your advice and pock out the holes and try vinegar it's less expensive. Thanks for the help, olliy

Don't just flush it. Fill spray bottle with vinegar and generously spray under the rim. Let is sit there for 1/2 hour or so before you flush. Repeat several times.