Why do I get lower framerate in Portrait than I do in Landscape? Can I fix this?

Hi folks.

I have been designing a game for Android that is playable in both portrait and landscape. I have an Android tablet that I am using to test my game.

I have recently noticed that in portrait orientation, a mostly empty scene runs at about 35 fps. However, in landscape orientation, that same scene runs close to 50 fps. With more objects on the screen, my framerate drops to 20-25fps in portrait while maintaining ~30fps in landscape, at which point it becomes acceptable only in landscape, which is very problematic.

What could explain this discrepancy in framerate, and what might I do to fix this?