Regarding the time it takes multi-tasking when in fullscreen mode:
I can confirm that it takes longer to multi-task between fullscreen applications when the native resolution in-game is different from the resolution set in Windows. Unfortunately this is an issue with how Windows handles multi-tasking with different fullscreen resolutions.
Regarding fps drops or refresh rate being lowered to 60 when multi-tasking in fullscreen mode:
The defect has been logged, as we seem to be changing the refresh rate back to 60 when a user multi-tasks back into the game client. This is mostly noticeable on high refresh rate monitors, such as 120 or 144hz.
To workaround the issue, you can disable v-sync. To do this navigate to: ‘Display & Graphics’ settings tab > ‘Display’ category > ‘Present Interval’ > select ‘Interval Immediate’ from the dropdown. This will prevent the frame rate from being capped to the refresh rate. We will be looking to address this defect in a future patch.