I'm actually pretty surprised no one's reported that mobs can spawn outside of the despawn radius before, unless I'm just blind (edit: I was)... It seems like the kind of issue that's probably been around for a very long time as well (just went and tried the earliest version of Minecraft in the launcher, 1.0, and I believe it's happening there too, just harder to notice due to short entity rendering distance).

Anyway, when this is happening over land, I'm not sure it's even really causing any noticeable lag as far as I can tell (might be why I didn't find a report on this), though it's still needlessly causing hundreds of mobs to constantly spawn and despawn every second. However, if you do this above oceans, it's much, MUCH worse - thousands upon thousands of sea life spawn/despawn every second (it'd possibly even be at least 10000 per second if it didn't lag), which slows the game's tick rate down by upwards of about 80% from my own tests (render distance 16, 200ish blocks in the sky, surrounded by almost purely ocean). The entity counter in F3 constantly fluctuates around 250-300, but you can also see it constantly briefly spike to around double those numbers. The majority of these entities are pretty much definitely fish due to how common they are and how many can spawn per group.

P.S. This problem is unrelated to the overall slower tick rate bug introduced in 1.13-pre5 (MC-132259), since this happens in earlier 1.13 snapshots as well (heck, more like this has probably been around since around the dawn of the despawn radius, but has only become a serious issue now due to the new water mobs haha).