Is it obligatory in this situation to decrease the speed of the vehicle in order to execute the passing manoeuvre safely?

Why?

The stationary truck restricts your view, and a pedestrian is using the crossing ahead of it. When passing a stationary vehicle, you must leave safe clearance and slow down whenever safety requires it. Here, you must reduce your speed and give way to the pedestrian on the crossing.

art. 23 ust. 1 pkt 2 oraz art. 26 ust. 1 Prawa o ruchu drogowym