http://monergiwmuoskoc43fj6vfbcrll5wjrbl647cdgg2zuwkbu77o7v5vyd.onion/interacting/monerod.html
Because the RandomX PoW used in Monero is very expensive (even for verification), monerod offers skipping these calculations for old blocks. In other words, it’s a mechanism to trust monerod binary regarding old blocks’ PoW validity, to sync up faster. --block-sync-size How many blocks are processed in a single batch during chain synchronization.