How can you scroll more smoothly: You can't. All possible speed optimizations have been already implemented. There is nothing that could make Sly more fast/smooth. Well, except rendering it in canvas/WebGL.
How can you scroll slowly in last to first item cycle: You could listen to 'active' event, and when the last item is activated you could change the 'speed' value with set() method to something bigger, and than on the next 'active' event change it back. But you are fabricating problems in here...