When you fly too fast (on your old maps from 1.12 and below)

-Chunks will disappear and reappear every millisecond and it causes lag

-Sometimes you'll be sent to the nether/end portal even when your very far away from it

-Sometimes pressure plates will activate while your flying

-Sometimes teleports you to another/2 or more chunks away from what direction you go

-Repeating command blocks (always active) with execute player commands will spam it's command all over the place (you can notice it with the playsound command)

I have a 4G RAM Android Phone and my old worlds run well, so lag is not the reason why this happens