I've found an issue similar to "looped belt too full to go at full speed" but is affecting non-full belts.
01-03-2025, 14-16-55.png (316.55 KiB) Viewed 157 times
As you can see, the outside loop is fully compressed from the bottom left on round to the single lane balancer, but not along the bottom stretch.
Reproduction:
Use the blueprint
Once the belt is loaded, stop the belt before the balancer by rotating it until items stop moving.
Restart the belt by turning it back to normal.
Wait around 8 seconds
Observe as it slowly compresses from the balancer back round.
Thanks for the report however the same thing applies in all of these cases. Looped belts when packed with items will eventually run into the "can't move at full speed because i'm putting things onto the back of myself and there's stuff in the way".
Belts were never designed to be looped, they "work" mostly, and won't crash, but you will always run into stuff like this.