Very high latency when "Always Defer Chunk Updates" is enabled

[bk5115545]

Bug Description

This issue isn't easy to reproduce but a couple people are having it in our Monifactory group. Monifactory is a Gregtech-esk modpack and our base is very dense so there are likely a large number of updates each tick.

MC: 1.21.1
CPU: AMD 7950X
GPU: RTX 4090
SSD: Samsung 980 PRO 2TB

I believe there may be a data race and a missing backpressure mechanism on the queues used to defer chunk updates. I run MC at 140 FPS and it stays pretty locked to 140 when "Always Defer Chunk Updates" is enabled however it drops down to about 120 FPS when it is disabled.
Rarely, Hammerlib starts to report between 2 and 50s of latency until I am eventually dropped from the server. I can interact with the server during this time though if there is 2s of lag then I have to wait 2s to see my changes. When I disable "Always Defer Chunk Updates," my FPS tanks down to around 70 and the lag starts to go down until it stabilizes at 33ms where my FPS recovers to around 120. After a while, I can re-enable the setting to go back up to 140.

Things I have tried that did not make a noticeable difference:
Java 19, 20, 21
Render threads from 1 to 16

Reproduction Steps

Be around a lot of chunks with a lot of updates? Still trying to figure this out

Log File

2025-02-04-2.zip