I guess you are using your mouse to adjust.
Holding down "Control" on the computer keyboard while dragging give you fine control. I'm on windows. On a MAC i guess it's the "Apple" you can hold down.
Also: If you use a controller and assign a control to the arp rate, then you can adjust in finer steps.
I tried the Control Keys with the mouse and it didn't have the desired effect.
I did try a MIDI controller and it does give a finer adjustment. Still in kind of
large steps but much better. I don't know why the normal mouse fine adjustments
don't work on this control.