Mailbird uses fancy animations when transitioning between different emails in your inbox and when expanding or collapsing messages in a conversation.

If you don't care for the animations you can improve the performance of Mailbird my simply un-checking "Show Animations" from the Mailbird Options menu under the Advanced tab.

To turn the animations on or off go to the Mailbird menu in the top left of the main window > Settings > Advanced Tab > Check or Uncheck "Show Animations" below Performance.