In particular, Mutter 2.31.2 has performance enhancements and theme improvements too. In the first post-2.30 development release, Mutter 2.31.2 adds a flexible version mechanism for themes, new theme variables, performance enhancements via streaming raw damage updates to ClutterX11TexturePixmap for partial stage updates when windows change, and various other bug-fixes. Mutter 2.31.x is also being built with GSEAL enabled, which is the macro that allows the compiler to catch direct access to object fields, so that developers will instead fix that to use the appropriate access functions. This is being done in order to facilitate the move to GTK+3 and cleaning up its API.
The complete list of changes along with the bug-fixes for Mutter 2.31.2 can be found in its GNOME announce message. Mutter is the window/compositing manager for the GNOME 3.0 desktop that is backed by Clutter to use OpenGL to provide a nice window management experience.