do any other implementations support something similar
Not that I'm aware of. But someone correct me if I'm wrong.
Is it fair to compare this ability to make fee/speed trade-offs to Bitcoin’s mempool where you can pay more to get your transaction approved faster?
Partly yes. But for Onchain the upper speed limit is waiting for a block to be mined. This can take time depending on luck and there is no chance to influence it. Even worse if you want the security of 3 or 6 blocks on top of it.
got it, thanks!
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