First, please try the suggestions in the If Apple Mail Is Running Slowly section.
A small percentage of customers using macOS Tahoe 26 are seeing slow training in Apple Mail that is not resolved by the general fixes. The SpamSieve menu bar icon changes color for an extended period of time before the messages move to the Junk mailbox or the messages change color but don’t move at all. If this is happening for you, please update to SpamSieve 3.2.2 or later and click here to enable an experimental workaround. (You can click here if you need to disable it.)
Please let us know whether this helped. If you need further assistance, please see the Reporting Apple Mail Slowness section.