When transiting an end gateway, the player will occasionally experience slow chunk loading and abnormal memory behavior for a period afterwards. My observations are as follows:

• During the period of slow chunk loading, the values next to "Mem" will "flush" very rapidly; I was unable to get such a high rate of flushing when generating new or loading existing chunks anywhere in the end. The values next to "Allocation rate" and "Allocated" will also remain fixed for the duration of the slow chunk loading; normally, "Allocation rate" updates every half second or so.
• The issue consistently fixes itself upon pausing the game, or when all chunks within the player's render distance have finished loading.
• The impact on chunk loading speed is highly variable. Sometimes things are a little bit slower, other times it resembles playing Minecraft with maxed out CPU usage. This seems to heavily inversely correlate with the displayed allocation rate.
• The issue seems to be specific to using an end gateway. I attempted to reproduce by simply teleporting myself to the exact locations where the gateway would normally deposit me (including rotation) but was unsuccessful.
• Render distance seems to matter. I was unable to reproduce at all on 12 chunks, could reproduce most reliably on 16 chunks, and could reproduce only once on 20 chunks. In all cases, my simulation distance was set to 12.

So basically it seems like the allocation rate is getting "stuck" somehow.

To reproduce

1. Go to a naturally generated end gateway (a "bi-directional" one, not a random one that teleports you back to the obsidian platform)
2. Set your render distance to 16 chunks
3. Place a full solid block 2 blocks north, 2 blocks west, and one block down from the gateway block itself. Then throw and ender pearl into the gateway and repeat this on the other side. This will make it possible to travel back and forth with just a click.
4. Go into third-person mode so you can more easily see your surroundings and tell which side you're on
5. Open F3
6. While standing on the placed block, throw an ender pearl into the gateway. Do this repeatedly until you notice slow chunk loading and abnormal memory behavior after a transit.
7. Observe that pausing the game or waiting for all chunks to finish loading will fix the issue.

Notes

I originally reported this issue as part of MC-272622, but was requested to make another ticket.

This occurs when using the default JVM arguments, as well as when using my normal custom ones.

I'm unsure whether this issue is gateway-specific. The allocation rate number does sometimes stay fixed when teleporting long distances in the overworld, but it's not clear to me that the chunk loading is much slower than usual when this happens.