There is a misconception about the "slow motion videos" in iPhone.
In fact, they *really* are videos recorded with a high frame rate (up to 240fps in iPhone 6).
They are *not* in slow motion per se - even though the iPhone software may play them as such.
In fact, I'll go so far as to say that there is no such thing as slow motion videos. There are videos played in slow motion.
What happens when, say, you drop your 240fps video in a 24fps timeline, is that it plays at normal speed. This is as it should be!
The big deal is that, since your clip has all those extra frames in it, you can use FCP X to slow it down (up to 1/10 the original speed in my example) and it will use all the real frames the camera captured. Plus, you can use all the retiming features in FCP X to obtain perfectly smooth slow motion.
By means of comparison, if you slow down a video recorded at the same frame rate as the project it is in, FCP X has to make up extra frames that did not exist. Even though FCP X has some great algorithms for this (like "Optical Flow"), it can't match real captured frames.