I don't think it's possible without slowing it down using a video editing application. The photos app doesn't have the ability to slow down time lapse videos since they're supposed to go at a fast rate. To show change over time quickly.
A timelapse video is, in essence, a series of individual frames put together into a "video". the data "in between" the frames is not there on a timelapse video. So you can't really "slow it down".