Video Editing

[anaxcrosswords]

I'm using Cyberlink PowerDirector on Win 10 to compile a montage of very short clips over a piece of music.
I'm downloading fairly long clips from YouTube then trimming them to the 3-4 seconds each that's needed, but playback performance is awful, like Cyberlink is struggling to process what's been written into the timeline.
Do you think it's because these very short segments are trimmed versions of much longer original files, so the software is still trying to read them in their entirety?
I can create the short clips as separate files then import them, but it's going to mean creating 60-odd clips - perfectly doable, but fiddly.