Comments above regarding where Libbitcoin achieves its performance advantage are not correct. The major performance advantage comes as a consequence of massive parallelism.
Milestone and assume valid are from a logical and security standpoint the exact same feature. Libbitcoin does not perform unnecessary checks under milestone. Core must build a utxo store even under assume valid, and this ordered building of the utxo set imposes strict ordering - preventing parallelism. It also operates as spend checks, but those unnecessary checks cannot be avoided due to the need to build the utxo set.