Chunk loading is slow, causing lag spikes, void walls, and lighting errors.

Chunk loading is slow, and can be random. The slow chunk loading can cause problems when flying with elytra or flying around in creative mode, this makes it very hard to traverse the world, as you are constantly hitting void walls. It has been reported on all platforms. 

Note: Void walls occur at around 2 chunks out of actually rendered chunks, not right at the border of an unrendered/unloaded chunk, and also block lag can also be a major problem, even at any simulation distance, especially on larger worlds and during autosaving.

Singleplayer

Chunks load slowly, this can be seen when entering a world on Nintendo Switch. Also occurs when flying in creative or using an elytra, causing you to hit invisible vertical walls in the unloaded chunks (looking like the void), hampering the gameplay experience. In addition to this, when you land at an area with lighting or shadows (eg: trees, torches), sometimes the lighting can look dark and unloaded, and you must wait a few minutes for the lighting to load in a return to normal.

This has been tested on Nintendo Switch and Windows 10 so far, but has also been reported on iOS, Android, Xbox One, realms and servers.

Host (Nintendo Switch)

Large 120MB survival world: SINGLEPLAYER chunks not loading.mp4
Fresh new worlds: SINGLEPLAYER chunks Not Loading Far.mp4 SINGLEPLAYER chunks loading slowly.mp4 SINGLEPLAYER chunks loading slowly 2.mp4

Multiplayer (Affects both host more than client)

This did not occur before v1.16.0 (the multiplayer client issue), so this update may be the problematic area.

Additionally, in v1.16.0, more issues with chunk loading were introduced in multiplayer, when more people are in your world, neither you or them could load chunks if you fly far enough, this could usually happen if the players were apart or in different dimensions. Before v1.16.0, these issues were less severe. The more players in the world, the more severe the issue is. Made less worse in v1.16.20 and v1.16.40, but can still occur during autosaving.

The problem occurs when autosaving starts, a variety of things can occur with the severity depending on your world size and simulation distance:

Block lag, chunks not loading (causing empty voids), lighting not loading, entity and mob jittering/lag.

 
Table for my large world

The duration of the autosaving also contributes to the severity of the lag, if the autosaving is short (less than 10 seconds) barely any lag happens, but if the autosaving is lengthy (1+ minutes) lots of lag occurs.

This occurs for all platforms (I've tested Windows 10 and Switch, other duplicate reports coming from iOS, Android, Xbox One, realms and servers), no matter who the host is (if applicable).

Host (Nintendo Switch)

That same large 120MB survival world (tested at 14 chunk render distance, 4 and 6 and 8 chunk simulation distance): MULTIPLAYER host nether chunks slow whilst mining.mp4 MULTIPLAYER host nether chunks slow loading.mp4 MULTIPLAYER host chunks loading slowly.mp4 MULTIPLAYER host chunks loading slowly 2.mp4 MULTIPLAYER host broken slow chunks and lighting not loading.mp4 three player.mp4

Client (Windows 10)

That same large 120MB survival world (also tested at 14 chunk render distance, 4 and 6 and 8 chunk simulation distance): MULTIPLAYER client not loading chunks.mp4

 

Additional Information

The multiplayer client slow chunk loading seems to occur with more severity on large worlds, no matter where in the world you are (being tested on a survival world being a size of 120MB), on smaller fresh worlds, you can fly out apart from each other as far as you want with chunks loading faster. Lighting errors more often on larger worlds.

Changing graphical settings (fancy graphics, skies and render distance) doesn't seem to make a difference but (see below)

Edit: These problems are slightly less severe with simulation distance set to 4 chunks (instead of 6, 8 or higher), but ultimately the game is still hard to play.

Here is the link to the 120MB survival world a lot of these tests were based around if you want to test more: Achievement Get Copy