Hi simplesimon87,
Thanks for posting in Apple Support Communities! There can be many factors as to what is causing the app to run slowly. If this seems to be a behavior that also happens in other apps, follow the guidance in this article to resolve the system-wide behavior: If your Mac runs slowly.
If it does seem to be isolated to just the Mail app, follow these steps to clear out cache and open windows:
- Quit the app from the Mail menu bar (Mail > Quit Mail)
- Press and hold the Shift key on your keyboard as you click to open Mail again
We also suggest updating macOS to the latest version. Outdated software can cause things to misbehave: Update macOS on Mac.
Hope that helps!