Storyline becomes VERY slow after importing video

What the title says.

I have tried importing couple of different mp4 files and .mov file (between 15-200mb) with same result in 2 different projects.

After importing the video that particular slide becomes so slow that I can no longer can work on it. Everything is delayed approx 10 sec. when i want to insert e.g. a text box.

Anyone with similar experieces and/or solutions :) ?

Thank you!!