I am not sure if it is appropriate for this task, but after using /editor command, you get access to an additionnal crafting menu that provide loader and infinity chest that provide a full belt. From that full belt you can use a self resetting timer couting to say 10, and allow a control belt to be active only when the timer is under 8, or 5, or 2 to choose the load. That may be not precise enough with those numbers as they are not multiple of belt speed so it create repeating pattern.
I have measured the belts throughput in the test to be around 33%, or 15 item per second, outside of the more dense area where it is around 23 item per second or 50 % throughput for that blue belt. Yet the UPS are not showing better results by the same magnitude, the not full belt may be 38% faster to update, it is carrying 66% or 50% less item.
Seeing this, i am suspecting that an empty belt would have better UPS that could be the control test .
I have use this contraption to measure the throughput and % but i've seen better ones :