The voiceover speed automatically adjusts based on the length of the subtitle display. By extending the subtitle length sufficiently, you can slow down the voiceover speed.
Reference: Voiceover Feature Spec
However, if the target step is short, the text cannot be shortened further, and the above does not work, here are alternative solutions:
Solution: Use Description Voiceover as an Alternative
About Description Voiceover
Use the Description Voiceover feature. With Description Voiceover, the reading speed is normal speed. The video will Repeat until the voiceover finishes playing.