There are definitely some scenarios in which the Zoom client or SDK using default UI will have superior video quality. This is mainly due to the fact that the default UI & client exist in an environment that we can control end-to-end.
We would like to implement a solution at some point that could allow the same video quality to be reached in a custom UI, but this is probably not something that will happen in the near future. In order to achieve the best quality possible with a custom UI implementation, I would recommend limiting the number of MobileRTCVideoView instances you have showing at any given time (note that this does not include your own video preview, as that is local video).