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.
Added debug screen screenprint (processor speed, java version, some chunk info, FPS, etc). Lag issue is very pronounced in this area. Perhaps the initial chunk conversion had a glitch? subsequent chunk loading seems to be ok- the sun and animals don't lag when other, more distant chunks were loaded into the game (i.e. the nether, more distant Overworld chunks).