As most 4th axis do, mine has a large gear reduction, 72:1 and further, a 3 to 1 belt reduction. When I am running code, the full rapids are at 100IPM for X, Y and Z, but the A axis moves at a snails pace.

I believe that Mach3 thinks that the A axis is moving at 100IPM, but because it is rotary and with such a heavy reduction, the rotation is crazy slow.

Oddly, when I am not running code and have the jogging feed set at 100, the A axis rotates at an appropriate rate.

Anyone have any ideas on how to fix this?

Actually Mach is seeing the feedrate in units/min which for the A-axis is deg/min.  As you noted 100 deg/min is very slow, (over three and a half minutes for only one rev!).
Mach can convert deg/min feedrate to in/min for a rotational axis. (It is a function of the radius).  On the settings page enter the nominal radius of the work in the A-axis ‘Rotation Radius’.  Touch off the Z on the surface of the work (to set the starting radius). Then during the cut as the radius (Z value) changes, Mach will calculate the correct feedrate in in/min.
HTH, al

As most 4th axis do, mine has a large gear reduction, 72:1 and further, a 3 to 1 belt reduction. When I am running code, the full rapids are at 100IPM for X, Y and Z, but the A axis moves at a snails pace.

I believe that Mach3 thinks that the A axis is moving at 100IPM, but because it is rotary and with such a heavy reduction, the rotation is crazy slow.

Oddly, when I am not running code and have the jogging feed set at 100, the A axis rotates at an appropriate rate.

Anyone have any ideas on how to fix this?