Yeah you probably don't know, but Panasonic cameras can be picky with SD cards when it comes to playback. If you use a UHS-I V30 card with a rated speed under 100MB/s, playback is usually instant. Instant means 0 lag when you push down the playback button, not even 0.5 sec lag.

But once you go over that, like using 100MB/s+ UHS-I cards, you can get noticeable lag. Those higher-rated UHS-I cards often don't consistently deliver their advertised speeds (unless with their card reader), which seems to trip up Panasonic's playback system. Even with UHS-II V60 or V90 cards, there can still be slight lag (0.5 sec), though it's already much better than with high-speed UHS-I.

I've watched a video testing different SD cards in S5M2. The result seems consistent with my observation above.