Hello,
timecritical things are all done by the controller (>>50000 steps per second are currently possible @18.432MHz) - the PC is only doing some preprocessing, basically the look ahead.
Every G-Code line translates roughly to 13 bytes of data + some communication overhead - so maximum capacity is about 750 lines per second for 115200 baud.
Sounds much but can be very critical in 3d and even 2d operation. If you e.g. follow a spline or arc that has been interpolated in 0.05mm steps you end up with 37.5mm per second in this example - already less than my machine usualy runs in wood. Of course this is also a question about wise or not so wise CAM settings - it is often not really necessary to interpolate things that precisely - but as I sell CAM software I know quite a lot people have the tendency to interpolate splines with 0.001mm precision and then wonder why their CAM is "soooooo slow" 
Running 38400 baud would be a possible solution if I limit minimum path length to larger values - but would be step backwards from what the control is already capable to do.
Christian