Mail now takes minutes to open an email. Problem is intermittent, but I've used Mail for years and this is relatively new. I have cleaned out my emails, and I have no antivirus programs. No intensive processes are running. What's wrong?

Press and hold a Shift key while launching Mail from Applications or the Dock. That action causes the Mail app to open with no message selected.

Release the Shift key when the Mail message viewer window opens.

Then, you might try rebuilding Mailboxes according to Rebuild mailboxes in Mail on Mac - Apple Support. You can rebuild any Mailbox including Junk, Trash, Outbox, Sent, etc. You might as well rebuild all of them.

If nothing helps "Safe Mode" may be instructive: How to use safe mode on your Mac - Apple Support.

How full is your drive?

When you say relatively new, how old is the problem? Operations over time tends to reduce disk space just for normal usage: updates, new apps, creating new user data and the like.

Have you created a different user, logged in with it and created a new mail account to test? Your current user settings might be causing the problem or it could be corrupt.

John Galt has the next best steps.

As far as Apple support, how do you know they don't? Isolated events do not define a situation. Just because it happens to you does not mean it's a wide spread issue. And just like security concerns, Apple works first on the most serious things and fixes them first. All corporations prioritize operations and this is apparently a low priority item.

Have you filed a bug report with Apple.

Have you tried a restart?

You can start by clicking the "Support" link on this page bottom left. Just below this post.

We are Apple Support Community; users just like you.