Hi, since Minecraft 18w43 I'm noticing lag spikes within the game.
The world (seed: -4689860770415580557) I tested this on has been newly generated with the specified snapshot version.
Steps to reproduce:
Place or break a block. (tested in Creative Mode with and without moving)
I'm using the default launcher options, thus Minecraft starts with whatever is specified as the default value within the Minecraft Launcher (v2.1.1462) on Windows.
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Can confirm placing/breaking blocks causes lag spikes. I removed the part about walking around causing lag; that is MC-137537.

Can confirm /setblock also causes lag spikes. Any kind of block updates causes lag spikes actually, might be related to the new light engine.
Also can confirm this 18w43c and can confirm this is client side, only FPS and not TPS.

Can confirm this only affects solid blocks. For example, tall grass and glass blocks do not cause lag spikes.

Can confirm it also affects non-solid blocks that emits light. For example, torches do cause lag spikes.

can confirm

Its the chunk Updates that is causing this lag issues.

This is happening for me on my pc in 19w14b. I'm hosting a server on the same computer and playing on the server via localhost.
Edit: Only noticed this lag spike when I break some blocks not place them.