First of all, please make sure you are adding files from the hard drive of your PC, not from any removable device (USB drive, phone, camera, etc.) or network/cloud storage.
If that's not the case, please also try disabling "Optimize HD clips" option within program settings -> Preferences -> Acceleration tab.
If you still experience lag when importing videos, please contact us while providing the following info:
1. Please right-click one of your input files on the timeline and select File Information.
Make a screenshot of this small window and send it to us.
How to take a screenshot: https://movavi.freshdesk.com/en/support/solutions/articles/36000375836
2. (Only for Windows) Please download the file at the following link: https://cdn.movavi.com/dl/support/GetMovaviLogs.exe
Launch it and wait for the program to complete the logs collection process, click OK once it's done.
The program will open a folder for you, please send the file movavi_logs.7z to us in your message.
For some more info regarding the technical side of such issue, you can also check this article: https://movavi.freshdesk.com/en/support/solutions/articles/36000071164