The bug

When flying with elytra, new chunks in front of the player generate very slowly and sometimes even don't show up.
The issue is present if the render distance is lower than 32 as well (for example 10 chunks).
Despite world generation taking up most of the computing power, but despite that world loading doesn't function correctly and cannot keep up with the player's flight speed.

As an example, here's a debug profile created in 20w18a with render distance 10 chunks: profile-results-2020-04-29_20.40.03.txt (by Kostas_GR)

This problem does not exist when the chunks have been generated before.