Depending on the neighboring blocks and the rate at which the supporting block changes, the scaffolding's distance blockstate updates with a different delay. In most cases, scaffolding's distance value updates 1 game tick after the supporting block changes. However, in some circumstances, the scaffolding will change instantly. This is most noticeable when observers are placed next to a column of scaffolding, and a trapdoor underneath is attached to a moderately fast redstone clock (3 ticks on 3 ticks off, or faster). This is especially frustrating, as observers are required to detect these updates to begin with!

I would expect scaffolding's distance value to propagate with a consistent amount of delay, regardless of the neighboring blocks or how the supporting blocks are modified. Without consistent delay, redstone involving scaffolding for signal propagation is limited to much slower speeds, despite observers being plenty fast enough to keep up.

Attached to this report is a suggested setup to aid in testing, and a datapack which can detect the delay at which scaffolding changes.