Chatting causes lag to occur

It occurs when you send the first text message on singleplayer world or on a hosted multiplayer server on 1.17.
The lag spike affects to everyone else, freezing the game completely for a second.
You can see a video of the bug here.

Can confirm in 1.17.1

It happens not only in chat, but also in displaying prompt texts when toggling mod functions (eg. tweakeroo, minihud, etc.), so it may be caused by text generation and rendering.

Besides, it is worth mentioning that the lag is even worse when Fabric is installed.

This is the result of profiling when chat lag and prompt lag occur.

Chat lag

Prompt lag

Looking the ticktime and CPU usage, when lag occurs, the ticktime is very high, but the CPU usage is relatively low. Therefore, I think a thread blocking may have occurred.

Chat lag: Tick 249:

[00] scheduledExecutables(1/1) - 98.28%/98.28%

[01] |   unspecified(1/1) - 100.00%/98.28%

[01] |   fabricClientEntityUnload(1/1) - 0.00%/0.00%

[00] updateDisplay(1/1) - 0.85%/0.85%

…

Prompt lag: Tick 70:

[00] updateDisplay(1/1) - 97.73%/97.73%

[01] |   unspecified(1/1) - 99.98%/97.71%

[01] |   mouse(556/556) - 0.02%/0.02%

[00] unspecified(1/1) - 0.96%/0.96%

[00] gameRenderer(1/1) - 0.74%/0.74%

…

When lag occurs, unspecified categories under scheduledExecutables and updateDisplay take up most of ticktime.

It seems that Minecraft Client is checking whether a player is blocked or not, this causes network request happen in the render thread. In bad network environment, the lag is way significant. On the other hand, if we tested the lag in offline mode, it will no longer exist. Here is a VisualVM Sampling result, you can see the Yggdrasil's HTTP API just blocked the render thread from handling user input and render the frame.

(used fabric intermediary names to debug and yarn names in blue marks above)

I used Mixin to skip the check, the lag I'm suffering is gone.

I think the HTTP Request should not happen in the Render Thread. The block list should be fetched either in other threads or before playing.

Sharing my findings from MC-235836

Inside the clientbound chat message packet, there is a "sender" field to identify the chat message's author with their's player UUID.

Upon sending a chat message as a player (or receiving one from another player), the client will freeze for about a second, likely for the reasons leedagee mentioned above. The client no longer experiences this issue for future chat messages that are sent/received, until it has been restarted (from my experience).

The issue however cannot be reproduced if the sender field is empty, e.g. system messages using tellraw or modified server software where the server simply doesn't send the sender field / sets it to a blank UUID on player messages. Oddly enough, this issue also cannot be reproduced on systems with AMD graphics. I've only been able to reproduce it on systems with NVIDIA graphics.

However, this can also be reproduced on my laptop with AMD R7 M360 GPU and my desktop with AMD RX580 GPU. I don't think it is an issue relating to specific graphics.

For me, this only seems to happen the first time the chat is opened, after that no lag occurs.

Can confirm in 21w37a.

Affects 21w38a.

Whispers (/msg) are also affected by this bug.