Fast voltage regulator for output of an function generator
This is exactly what a function generator is meant to do. The obvious solution is to figure out what takes time when sending a command to the function generator – it really should take milliseconds.
If it's some stupid driver issue in labview: most function generators that can be controlled from a PC listen to SCPI commands, and there's libraries to generate these. It's really easy and quick to just change the amplitude of a function generator.