I tested the simulation interface. It seems to work fast.
I think the speed of interchange can change depending the direction of exchange of data, from or to simulink sink.
Have program a realtime simulator for powerplants written in C++.I translates the RT sim to NI components and software (Labview).
My rt pxi turbine simulator for simulating grid incidents was succesfully used in a nuclear plant in 2006.
Look at http://sine.ni.com/cs/app/doc/p/id/cs-755