your ASIC that is actually mining the blocks doesn't validate blocks or construct next ones, it just hashes the 80 bytes that it receives. while it is hashing, your node can easily construct the next one before it reaches the end of nonces and fails to find and requires a different change in extranonce or something. and while you are at it you might as well add new transactions and construct the merkle root too. and since this is done elsewhere (not in ASIC) then there is no difference in time it takes to mine an empty block or a block with 3000 transactions!