Blizzard has posted an update on the Dragonflight performance issues, clarifying that operations and server engineering teams have been working around the clock to fix them since the expansion launched.

(Source)

Since the launch of Dragon Isles content, our live operations and server engineering teams have been working non-stop to address several unexpected performance issues. Some of these were fixed with realm restarts that took place earlier today in all regions, and other performance issues are still being worked on.

This is a top priority, and we expect to deploy additional fixes over the next day.

Thank you for your patience.