Bitcoin Core – Why is there an “increasing progress per hour”?

To answer the question I think you have:

“Progress increment per hour: 0.25%” means that 0.25% of the blockchain will be synchronized per hour, or it will take 100%/0.25% = 400 hours to synchronize the entire thing from scratch (assuming current speed is constant).

That’s right ~ no What it means is speed Synchronization is increasing over time. It will generally fall.

To answer your question word-for-word:

There are many reasons why progress may be inconsistent. Progress is the ratio of the total number of verified transactions divided by an estimate of the total number of transactions on the chain. However, it is as follows:

1. Not all transactions are equally complex, and the composition of a typical transaction has changed over the life of Bitcoin.
2. Signature verification is only performed after the presumed validity point, which is likely the last few percent of the chain (unless you manually configure it with something else).
3. The size of the UTXO database increases dramatically across blockchains. Larger databases operate more slowly. This is especially true if the database no longer fits into the in-RAM cache.
4. Transaction count estimates may become slightly more accurate as the chain progresses.
5. The peer being downloaded can change, and sometimes the peer providing the data can quickly go offline. Sometimes slow peers get kicked out (this is mainly important if there are bottlenecks on the network side).
6. Your computer may be performing other tasks at the same time, which may change.