I should add here that the bike has normal v brakes (someone asked about coaster brakes) and that one noticeable difference from the previous, smaller 16" bike is that this one is heavier, so naturally it was gonna be harder to pedal compared to the previous one.
However, I just wanna see if the riding experience can be improved, hence the slow wheel spinning question.

Thank you all for the suggestions. I would like to try to open up the bearings and relube them but have no idea on how to or even if they can be opened. I wish I could post a picture.