He wants to enjoy Minecraft by complaining about the lag more...I guess. Most people seem to think that the Bedrock version is the much-better coded platform and about 2 years ago or so they experimented with bigger worldsizes. It did not end well.
If they are referring to increasing the world height there shouldn't be major issues as long as world generation is not changed and the height isn't increased too much, much as 1.2 doubled the height from 128 to 256 but terrain wasn't changed (until 1.7 but even then terrain very rarely exceeds 128 unless you use Amplified), and performance and memory consumption actually improved since instead of loading entire chunks the game now only loaded 16x16x16 sections that had data in them, with the average loaded section height being around 5-6 based on typical terrain, 4 over oceans and 1 in Superflat.
A more ideal solution would be something like "Cubic Chunks" (an actual mod that gives "infinite" height), with chunks completely broken down into "sections", including a vertical render distance (currently entire chunks are loaded and rendered at once, even at 2 chunk render distance the game will load and render blocks 256 blocks/16 "chunks" above or below you; a 5x5 chunk area 256 blocks deep is like a 10x10 chunk area 64 blocks deep, at least with regards to memory consumption as the render load is mostly based on the number of visible faces being rendered). Of course, this would require an extensive rewrite of much of the game (rendering, world generation, chunk loading/saving, mob AI and spawning, etc, basically anything that currently assumes chunks go from 0-255), and there are still issues with light updates with great heights (the game needs to recalculate sky light changes all the way down to the ground, including accounting for unloaded chunks).