AT-SPI2 is significantly slower than at-spi-corba and so is not really usable.
Currently, the main issue is that pyatspi2 is written entirely in Python and uses the dbus-python bindings. Having a C core with gobject introspection should improve things significantly.
Having peer-to-peer dbus connections should also help. There is currently a branch called p2p for this in at-spi2-atk and pyatspi2. I had not merged this with the main branch because it needed a dbus-glib fix (FDO#30574). This fix is now part of dbus-glib 0.9.0.
Both of the aforementioned changes are now in the released AT-SPI, and AT-SPI2 is significantly faster now, so I am going to mark this bug resolved. However, method calls are likely still slower than they were with AT-SPI-CORBA, while some tasks (such as retrieving an object's children or states) are likely faster because they rely entirely on cached data.