With Emmy* the block times will be 30 seconds (if blocks are at priority 0 and they have at least half of all endorsements).
With Tenderbake, the block times are given by the duration of the first round, which is a parameter of the algorithm, and is not yet fixed.
One thing to keep in mind is that block times should be bigger (at least most of the times) than the time to propagate blocks between any two nodes. Currently, a block is propagated only after it is validated. With the current gas limits per block, validation may take up to 10 seconds, when the block is “full”. Given that a block might need 4-5 hops to propagate over the p2p network, we are forced to set block times bigger than 40-50 seconds. (That is why Emmy* in order to reduce block times from 60 seconds to 30 seconds also halves the gas limit per block.) To improve block propagation times, a direction that is being explored is to remove the dependency on validation. That is, a node could propagate a block before fully validating it.
