You could be using HEVC video compression to save your videos. Many systems do not support this format. Swipe down the screen- preferences- general- video compression. In case it's set to HEVC change it to H.264+HEVC. How this helps.
I have one ..
1. What settings did you use to shoot ?
Try shooting in different resolutions to know where the issue is.
2. Try playing in Quicktime 3. If it's a 4k 30fps, it might just play only on Mac. If it doesn't play on a Mac, it's a video issue.
4. The lagging is also at times because of the memory card in the card. Format it and start fresh.
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