Dan Newman has been making lots of changes in "ReplicatorG-Sailfish", an enhanced fork of ReplicatorG that support Makerbots running the "jetty" or "Sailfish" firmware that Jetty and Dan have created to improve the performance of the printers.
One of the nicest improvements is the incorporation of PyPy support to speed up slicing significantly.
One important thing to note: because of differences between the Makerbot "stock" firmware and the Sailfish "aftermarket" firmware, you must use the appropriate ReplicatorG matched to the firmware you are running on your machine. If you run the stock firmware, you can also run Makerware.
If you have a Makerbot and would like to crank up its performance, be sure to give the group a good read. With a little bit of investment in time, you will gain a lot of performance!