It's the consensus algorithm. Every transaction on a blockchain is verified by differents points.
As you can see on blocktivity.info blockchains like Ethereum or Bitcoin are almost at full capacity, since the transaction confirmations are done during the mining process, that's called Proof of Work.

Blockchains like Steem and Bitshares operate under a Delegated Proof of Stake algorithm (DPOS), much faster and with much more capacity.