Just did some tests in the shop. Setting the Radius in the Settings tab did absolutely to affect the time of a coordinates A/X move. I did increase the motor speed to 1500 from 200, which greatly increased the rapids, A 45 degree 1" line on a 3" diameter workpiece took 8.5 minutes at F15, a move that takes 4 seconds on a flat plane. It seems to me that Mach3's trajectory planner is not working properly here. If I change that move from G1 to G93 G1 F15, then the move does take 4 seconds. However for engraving, all the moves are very short, so to use G93 each G1 would need a different F word based on the move's length. The CAM wrapper I use doesn't support this conversion.
Unless I can find a Mach3 setting to coordinate the axes better I am at a standstill.