When you open a video file (MP4, MKV, MPEG, etc.) in macOS, is GateKeeper (macOS Security) attempting to “verify” the file on first open? Run this in Terminal:
1 defaults write com.apple.LaunchServices LSQuarantine -bool NO
All video files in macOS will now open without being quarantined and verified. Keep in mind If you run future macOS software updates, you made need to re-enable the Terminal commands above.