and welcome to the forums.
Can you confirm that you are using Corel Video Studio X4, if so I will move your post to that forum.
When you say “Video Portion” do you mean your video and not the Instant Project clips.
If your video file is HD then you may need to use a process called Smart Proxy
This is as I remember Enabled by default.
When you add a video clip to the timeline proxy files are automatically created, these do take some time to create, be patient.
Once created the timeline playback should improve.
View Settings Smart Proxy Manager and Queue Manager.
Please view this post for image?viewtopic.php?f=1&t=43859&p=231968&hilit=double#p231968