Severe lag in 1.12 save game, in 1.13 Minecraft session

Loaded a v1.12 game into the V1.13 player.  There is a severe lag problem-  animals are lagging, and the sun moves across the sky, then 'steps backward', then moves forward, steps backward.  [I've seen this problem in the old Xbox 360 version, but my laptop is much more powerful, and has never occurred before]

Oddly, the problem only occurs in the area where my minecraft character was located, when the game was first loaded by the new player.  i.e. after stepping into a nearby nether portal, and emerging into a different section of the Overworld (over 1,024 blocks away), the lag problem is not present.  [e.g. the sun is 'hitching' in the one area, but progressing smoothly across the sky in the new area].

Both areas are ocean biomes, and aren't overflowing with excess mobs, so there's no overt reason why a lag should be occurring.   My laptop is powerful enough to run an amplified world without much of a slowdown, so I don't think it's a hardware issue.