Over several years, I have accumulated some 1400 messages in the SENT folder. Lately my Thunderbird is taking a long time for some actions that used to be instant, like selecting another message to read or delet. I regularly remove incoming messages to keep the in-box under 300, and I have almost nothing in the other folders. I tried changing the "local folders" to a different directory, but when I do that, all the old "sent" messages become un-available for searching and potentially continuing a conversation.

Is there a way to move the large number of "sent" messages from the "local folders" but still keep them available for access from within Thunderbird, in case I need something from past chains of replies?