The structural lag
Excursion metrics need the trade to be over before they exist.
Maximum adverse and maximum favourable excursion are properties of a completed trade's whole life. Until the position closes, the maximum has not happened yet — any figure computed mid-trade is provisional and can be overwritten by the next hour. The measurement is therefore inherently retrospective, not because the instrument is slow but because the quantity being measured is not defined until the trade ends. This is a permanent property of the analysis, not a limitation to be engineered away.

