Is the driver obliged to apply the so-called "zipper merge" when there is no significant speed reduction ?

Why?

Mandatory zipper merging applies when traffic has slowed significantly and a lane ends or is blocked. Without that significant slowdown, the driver changing lanes must give way to vehicles already in the destination lane. The directional arrow shown requires a lane change but does not itself give priority.

art. 22 ust. 4, 4a i 4b Prawa o ruchu drogowym