There were various things like adding support for the new gtk dmabuf import APIs for zero-copy playback, however most relevant for mobile was IMO the rotation support.
As mobile cameras produce rotated content we need to rotate somewhere in the pipeline. Previously we did that with the gst videoflip element - an extra copy, in software. Now that step is essentially free.
https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1590