My understanding is that the original intent of Servo was to be a way to develop features and port them over to Firefox itself (which did happen with at least a few features), and the relatively slower pace of developer is more due to Mozilla laying off everyone who was working on it. (Yes, presumably many of the same people are involved, but I would expect that being able to work on something full time without needing another source of income will end up making progress faster than needing to find time outside of work and balance between other things in life, ideally in a way that avoids burnout).