Why is composing messages suddenly painfully slow?
Composing email messages in Thunderbird has suddenly become painfully slow. Rebooting my computer (iMac running OS X Yosemite) and uninstalling and reinstalling Thunderbird has not helped. When I type words into the message window, I can't see what I did until I hit command ` twice to go to the main window and back to the message window. This makes it take a very long time to compose even a simple message to send.
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Not a Yosemite Expert but that appears to be a focus-stealing issue on Mac OSX itself. The clicking _ ` _ is manually refocusing the 'main window' which I'm understanding from your post to be the ``Compose Window`` but as to why it is lagging does not seem to be Thunderbird's cause, consider looking at disk i/o during a replication session, and/or with a clean profile:
Clean profile being you copy/backup your present config `folder` and move outside its present directory and reload. If it persists it's not a user config issue if not it is something in your present config that has corrupted or misconfigured itself.