It is in the fountain fill dialogue box, there is a little lock so the 256 steps show grayed out, but if you unlock it, you can change this setting. It also appears on the Property bar when using the Interactive Fill Tool (flyout below the regular fill tool), again, remove the lock and you can edit. Some versions of DRAW only let you edit the steps in the Fountain Fill dialogue box. This box is available from the flyout from the standard toolbar from the Fill Tool, second button from the left or on your F keys, F11.

Impact on DRAW's performance, not much.

Impact in Xara, even less.