Can anyone help me with this lagging bug?

Hi all.

So, I have Premium. I had no problems using it until the first time I tried editing a nearly hour long video. Editing the video, I had no problems until maybe a third of the way into it. The video playback and the video strip at the bottom started lagging really bad. It got to the point where I couldn't work with it because the lag was so bad. I saved the draft, and went on to other videos. It's not the app, because shorter videos edit just fine.

I've tried to go back to the draft, but the lag is still there.

I would like to know what it is because I do have a few long videos on my channel. I would like to know if this is a recurring bug. I also don't want to lose the work I already did.

If anyone can help, I would appreciate it!

Thanks!