I did self-paced work my first year and regretted it. The new and slower students start feeling overwhelmed when they see their neighbor at lesson 5 when they are still in lesson 1. The discrepency kept increasing as we reached harder concepts.
I have students check in with me at various points in the lesson and have them stop till the others reach that point. The experienced students are tasked with helping other students if they are done or read some related material. But their down time is not much since the check points are done every few bubbles.