TL;DR
Proof of History is Solana’s innovation that creates a cryptographic timestamp for every transaction, allowing validators to agree on time ordering without waiting for consensus.
In most blockchains, validators must communicate with each other to agree on what order transactions happened. This communication is slow and limits throughput. Solana’s insight: if you can prove that a transaction occurred at a specific point in time, validators can order transactions independently without waiting for others. PoH provides this proof.
PoH runs a continuous SHA-256 hash chain where each hash depends on the previous one. Since SHA-256 can’t be parallelized, the chain proves that real time has passed between hashes. When a transaction is included, it’s mixed into the hash chain, proving it existed at that specific point in the sequence. This creates a verifiable, ordered log of events without requiring consensus on timing.
Because PoH timestamps transactions before consensus, the leader can stream ordered transactions to other validators in real time. Validators can begin processing transactions as they arrive, rather than waiting for a complete block. This pipelining is why Solana achieves 400ms block times and can theoretically handle 65,000+ TPS. Without PoH, achieving this throughput would require much more inter-validator communication.