Do you mean P2Pool the decentralized mininig pool? If so, i'm sure it refer to how often the "share" is updated.
I'm a bit confused about that as well. If it chooses a 30s 'block time', that's just the speed of the
share chain. It obviously needs to have a difficulty that is much lower than the actual blockchain diff so that miners can regularly submit valid shares.
This doesn't mean that a valid extension of the actual blockchain is found much more quickly, it just means that n'*20=n. Stale blocks aren't a big problem in such a share chain.
Yes, that's what i wanted to mention since the change doesn't happen on Bitcoin on-chain layer and IMO it's different from what OP asked.
Arbitrary choice, it's unlikely satoshi performed research about effect block propagation on different block time.
Partly that, but in the whitepaper, Satoshi did mention it as a measurement for calculation of RAM/disk space requirements. It can be that as arbitrary as "10 minute block time "is believed to be, it might really be based on the current hardware available of that time when he wrote the whitepaper.
It's plausible explanation, but difference between best case (block header with no TX) and average storage/RAM in 2008 is too wide.