In 21w15a whenever you cross a chunk border on a world with the C&C world generation datapack, the game lags to 0fps for about a second on each border. I've tested it on both servers and singleplayer, and it happens on both.
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I've tested with render distances of both 15 and 3, the lag persists. I noticed that when this happens, the RAM usage sometimes goes above 2GB, and even with 4 and 8GB, the lag persists. I don't know what the issue is, but it makes the game very unplayable. All I can guess is that it has to do with a weird rendering or entity bug, because the world has already been generated in the chunks I'm lagging in.

I added a video of it happening with the f3 menu open, I notice that on the left side in the second grouping there is one labeled "SH S", and the numbers displayed on that line go to a "?" when crossing chunk borders.

Java version jre1.8.0_281 I was playing on local multiplayer with my father yesterday with the world generation datapack and I had experienced the same issue. It caused the lag down to zero with every chunk border crossing as well.

Java 1.8.0_281, Minecraft Java Edition 21w15a
Using the data pack results in 0fps and ram usage spikes at chunk boundaries making the game unplayable.

Update: It seems that when you go quite a bit farther out from the spawn chunks, the lag spikes aren't nearly as bad - quite odd. However, the big lag spikes still sometimes come back even when farther out, they're just not as common anymore.
Quite an interesting bug, honestly. I'd love to know what's causing this!

Oh my, I thought this was just me! I created a bug report for nothing it seems. +1 though! This really does need to be fixed!

Note: After some looking into this is actually a duplicate of [MC-197616 Certain custom biome settings cause game to spam "Received invalid biome id: -1" in the console, causing major lag or freeze|https://bugs.mojang.com/projects/MC/issues/MC-197616?filter=allopenissues]. It's just that it's indirectly, as both are about the same issue of world generation through datapacks.
Most likely a duplicate of MC-166005.

They may be related in some way, so I will be watching that one. Thank you. This ticket, though. is specifically about 21w15a combined with the C&C Data Pack.
21w14 didn't have 0fps and large ram spikes on my modest laptop.
21w15, by itself, doesn't either.
It's only when the C&C Data Pack is added in that I see the issue listed in this ticket.

Never noticed that the issue was over chunk boarders, but I can say that I too have this issue. Thought it was just me though because it doesn't start as soon as the world is loaded, but instead after a few minutes. Can confirm I had no issues in 21w14a either.

Still happening on 21w16a.

Duplicate of MC-197616

I haven't tested this much yet but with the new datapack from the 21w16a page I was not receiving any major lag spikes with crossing chunks anymore. It is not listed as fixed in the changelog but maybe it was patched? Again, I have only tested this for ten minutes, I have not tried any older worlds yet.