iphone SE to HDMI resolution and fps

I am running Zwift on my iphone SE. It is very stable.

The last two weeks I have been using a lightning to HDMI converter and the FPS is not as high out from the converter as on the iphone screen.

I estimate the converter FPS to about 15-20.

Is it possible to change FPS (and also the resolution)settings in the app?

Or is the converter the limiting factor? It claims to be able to handle res.1080

and FPS 60.