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. The client no longer experiences this issue for future chat messages that are sent/received, until it has been restarted.

Steps to reproduce:

1. Launch Minecraft & enter a singleplayer world (or vanilla multiplayer server)
2. Simply send a chat message, the contents do not matter.
3. Your client will freeze for a second as long as you have an NVIDIA graphics card.

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. It has so far only been reproduced on systems with NVIDIA graphics.