I think it varies from series to series. Some transformations (like mine!) happen within an impenetrable sparkly bubble, others may happen within a split second to everyone else but are a full sequence to the girl transforming.
I noticed in Cure Happy's transformation sequence, the pose she's in right before she transforms is the same pose she's in afterwards (and it's not the pose that ended the transformation!), so I think the Smile PreCure girls have "minute inside, second outside" transformations.