With 3D surfacing tool paths you won't get arc fitting unless the paths are in one of the major planes (XY, XZ, YZ). It's not a Fusion problem, it's a gcode/machine control problem. Higher end controls have very fast block processing speed, which means that paths with lots of tiny line moves are not a problem. They just get smoothed out in the control. It's only on lower end machines (including older industrial machines) where it becomes an issue.
So in your particular case it could just be that the control can't process the code fast enough and therefore never reaches programmed feed rate. But it could also be that the path created by the line segments is not as smooth in one tool path vs another. Specifically I've found that scallop tends to make more jagged paths than other strategies. There is a way to possibly improve it though. Can you post the file with the spiral and scallop paths in it?
C|