I suspect that you are trying to edit 4K video and your computer is struggling to decode it quickly enough. I recommend that you try Proxy Editing and make sure you set your Video Mode to be no higher than 1080 resolution.
I suspect that you are trying to edit 4K video and your computer is struggling to decode it quickly enough. I recommend that you try Proxy Editing and make sure you set your Video Mode to be no higher than 1080 resolution.