Ubuntu 13.04 improves graphics performance to help phones and tablets
A big part of the 13.04 upgrade has been a much smoother Unity interface, and better performance. A lot of that is based on the improvements made in an effort to make the OS better for "lightweight systems", which is the initial aim for Ubuntu Touch. Canonical said about the update:
Ubuntu Touch is planned to start life in emerging markets on low-cost handsets, so this update is laying the foundation for much better performance on those devices. This is still early work though, because Ubuntu Touch isn't planned for wide release until Ubuntu 14.04 comes out in April of next year.
source: Ars Technica