I guess that will depend on your machine and the size and kind of Brush tip you use.
With large Bristle Brushes I do get somewhat of a lag.
But maybe you should ask over at
or just try the public beta.
Still seems pretty slow.
But keep in mind I'm not testing with a system that can take advantage of all the possible GPU enhancements such as OpenCL (though I have no knowledge that the Mixer Brush CAN use the GPU in that way).