Drifting the thread a bit (nothing to do with englishal's lance), What is a 'slow' and what is a 'fast' aircraft?
I strongly suspect a lance can fly downwind at (say) 80 kts, perhaps with some flap, and I suspect most 3-axis type microlights can go quicker if they try. (I have no experience of flexwing).
Common sense would suggest we try to fit in with each other. If you're not pretending to be a 747, you can configure and pick your speed whenever appropriate. The microlight with a lance up it's chuff doesn't need to slow down until (possibly fairly late) final, the lance doesn't have to fly flat out downwind.
I was taught to alter speed to fit in. Always seemed more practical to me than have everyone heading all over the place / off downwind to ensure spacing. Makes spotting traffic much easier too.