This problem may be dependent on your machine controller's specifications. What kind of machine? How are solitary A axis feedrates to be commanded: what units?
Typically, simultaneous XA moves will be controlled by the X axis linear feedrate (both axis must start and stop at the same instant), but A axis movements may need their own feedrate command, and it might have a degree/min requirement.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)