I'm truly lost about how bitcoin works.

I understand a blockchain contains all transactions ever made. So, what things need to be verified while I'm downloading the whole blockchain? If it's just the hash of each block, the processing/verification speed of earlier blocks should be same as later blocks, right?

Only better guess I have, is that if anything need to be indexed, then that would explain why later blocks are slower. So is this the case?

2 Answers 2


Earlier blocks had fewer transactions, and so were smaller. Later blocks have many transactions, and so are much larger.

  • But, I thought I only need to verify one hash per block(?) What would I need to verify per transaction? Are you saying I need to verify one hash per transaction? Also, I understood that each block has its size capped at few Mb.
    – Sida Zhou
    Apr 7, 2013 at 13:10
  • 1
    @SidaZhou There are two major bottlenecks in synchronization. One is signature verification, and the other is looking up the inputs on disk and marking them spent. Both increase when transactions increase.
    – Nick ODell
    Apr 7, 2013 at 16:21

The hash verifies that it's part of the chain but it doesn't verify that it's legal. For example, it might contain fake transactions that invent money and send it to people.

Well-behaved software needs to check this, too. I think that the download time is what dominates the slowness, not checking the transactions.

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