When creating worlds with Custom World Presets, they take a very long time to initially load. After starting, it takes a very long time to load new chunks. This performance drop affects the ability to play, even in creative.
Please note the specs above, as 3GB are allocated just to Minecraft/
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If this issue is closed, it needs to be reopened. Please see here: MC-53584
There are errors in console and debug profiles. This is a problem that can be reproduced, and slows my 3.7 GHz 8 core cpu to much more than a crawl, as you say.
The "specs above" don't include your CPU, but I assure you, I can come up with settings that make even my machine at work slow to a crawl. Depending on the presets you're using, the game has to light a TON more blocks, has to propagate light in ways that just don't occur with normal terrain, and render more chunks in general. A complex world being more taxing on your CPU is absolutely not a bug.