SteamOS 3.5.16 Preview is available now for testing on Steam Deck, as Valve continue improving performance for the recently released Persona 3 Reload.
SteamOS 3.5.16 Preview simply has a one-liner changelog that notes "Further improved performance in Persona 3 Reload". Confusingly, updating to it in Preview didn't actually change the SteamOS version from the recent 3.5.15 Preview. I was already on 3.5.15, and it gave me the update. So it's possible Valve forgot to update the actual version string.
To test it out go into Settings > System > System Update Channel and pick Preview from the drop-down box.
You can buy Persona 3 Reload from: