Allow the project to fully cache, with the green line complete above each track's video clips.

Also, under the preview window, the ribbon should change color if there are multiple tracks. Normally, this changes from blue to purple.

This can take several minutes. But once cached, response should be good - if the hardware is up to the task. Large, high-def files extract a heavy load. A gaming-class PC is recommended.
Install the current 64-bit VP version. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
When starting a new project, go to OPTIONS | DISK tab and Clear Unused Cache Files. This can also be done periodically during an edit session. Do not load clips that won't be used in the project. For UHD content, proxies may speed up importing and editing. It's a common technique.
See https://nch.invisionzone.com/topic/26563-proxy-editing/?do=getLastComment
If you'd like to share, follow these instructions and someone will compare their results with yours. Or send me a link via Personal Messenger (envelope at top-right of this forum).
Back up --- With your project on the timeline, click on MENU at the top-left. Click FILE|BACK UP PROJECT FILES TO FOLDER. Choose a folder and SELECT FOLDER.
Upload --- Use a free server - Google Drive, MS OneDrive, etc.* - to upload the saved, numbered FOLDER. Do NOT upload the individual VPJ or export file.
Get link --- Get a public link. If using Google Drive click GET SHAREABLE LINK. If necessary change "restricted" to "anyone with the link can view"
Share --- Click COPY LINK | DONE. Paste that link here, or click the folder at the top-right of this forum to message it privately to me. It won't be shared.
* Before uploading, right-click the folder, click PROPERTIES. Look at the File Size to confirm that it's not too big for the free space on the server.