Why is the crowding-out effect of investment subsidies the smallest?

Because investment subsidies are short-term behaviors.

The crowding-out effect in economics is aimed at government departments, which refers to the result that the increase of fiscal expenditure leads to the decrease of private consumption or investment. That is to say, in order to balance the budget and deficit, the government uses the central bank to issue treasury bonds as a means to raise the funds needed for public expenditure, which makes the market interest rate rise and the investment and consumption of the private sector fall, which is regarded by the government departments as "crowding out" the private sector.