I don't understand how a transaction would take more time to propagate than an entire block filled with thousands of transactions?
probably compact blocks BIP152 plays an important role
because all blocks are composed of valid UTXOs which are relayed as unconfirmed transactions before inclusion in some subsequent block, most nodes do not need the megabytes of transaction information, as they already have all (or most) of the transactions in a given block in their mempool. Compact blocks propagation is designed to leverage this fact; nodes relay only the block header and hashed transactions to other nodes, instead of the complete block. If a node does not have in its local mempool some small number of the unconfirmed transactions in that block, then they request the full transactions they are missing. Then, the node uses the hashed transactions (now that they have all of them for that block) to reconstruct the complete block.
This reduces block propagation time (and the bandwidth used to propagate blocks) considerably (and has been in use since Bitcoin 0.13.0)