When monerod is initializing, it says
Loading precomputed blocks: 1200001.
What are precomputed blocks? Where are they loaded from?
This wording is refering to a list of known block hashes embedded in the monerod binary. These hashes correspond to the first N blocks in the Monero blockchain, and are intended to allow the daemon to skip some expensive checks for "known good" blocks. If your daemon receives a block at height H, and that block's hash matches the "known good" has for that height, then a faster verification is made.
This is in a way something that goes against decentralization, since the list of known good block hashes is made by the Monero team. However, the speedup it offers to first time users is substantial, and the known blocks are only for historical blocks (I think till 1 million at the moment, and we have 1.2 million blocks).
This system can be avoided by not building in the block hashes when building. Syncing will then do the whole set of checks from the start, and also be much slower, but it allows you to verify that you indeed have a good valid chain.