Are you obliged in this situation to reduce your speed?

Why?

The visible A-30 “Other danger” sign requires extra caution, but does not by itself require you to slow down every time. If your speed is already safe for the conditions shown, you may maintain it. You must slow down when the road situation requires it.

§ 3 ust. 1 rozporządzenia w sprawie znaków i sygnałów drogowych; art. 19 ust. 1 Prawa o ruchu drogowym