Why is the icloud download of videos to imovie ios so slow?

I shot a couple of videos with my iPhone (4k 60fps). The videos were uploaded to icloud (which was also very slow). Now when I go into iMovie I can see the videos marked with a cloud, but when i try to download them to my ipad to create a movie, it is extremly slow (5sec video takes 15 min to download) despite having a fast internet (120mbit/sec). Sometimes i even get an error message, that the download failed. What is going on?