We’re trying to build on Bitcoin Testnet4

Testnet isn't meant to "build on".

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ckpool is making it nearly unusable.

What makes you think it's ckpool that's doing this?

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You can see it for yourself — the same address has been collecting block rewards for 2 months:
👉 https://mempool.space/testnet4/address/tb1qum986qkqf363jhaau2nlavyehdqg487p8m2ydu

You may want to read Testnet4 Mining (Discussion) (to start):

The areshole who is abusing testnet4 is better with mining empty blocks to avoid any block being invalidated if any transaction was not right

I would have guessed it's because smaller blocks can be broadcasted faster.

This is incredibly frustrating for the developer community. Either this gets fixed, or we seriously need a new testnet.

I've seen some discussion about how to improve testnet, but all suggestions have drawbacks again.