Problems with 1080p HD video and iphone 5S

I have the iphone 5s. I edit videos that usually are lower quality, but recently i have found access to 1080p HD videos to edit. When i went into the app to edit these videos, it continuously froze up, became grainy and discolored, and in most cases just completely blacked out. The videos are only about 1 and a half min long, nothing huge.