Shozikit
Let us start with the full properties of your source media - either from a video audio properties readout program such as MediaInfo (Tree View) or from knowing the brand/model/settings of the camera that recorded the video. This information is necessary even though you state that your project settings match the properties of your source media.
http://mediainfo-mac.en.softonic.com/mac
Be careful in your download and install choices to avoid unwanted carry alongs with the MediaInfo program.
Is your computer optimized, even as brand new. See ccleaner Mac
Have you looked at program performance with and without the antivirus disabled?
Let us start here and then decide what next.
Thank you.
ATR