"(1) HL employs a custom VM and consensus mechanism principally optimized for trading as outlined earlier"
The features that are available don't seem to be that radically different between the 2 but Hyperliquid definitely looks to be the more performant of the two in terms of throughput and latency (expected given the differential in the validator count).
Each validator on the dYdX Chain runs an in-memory order book locally that is not committed to the chain (i.e. each validator is not expected to have the same version of orderbook) which is interesting but also required dydx to work on some anti-MEV measures. Whereas with HL all validators maintain the same copy of the orderbook.
The defining feature for both to attract market makers was priority for cancel orders over the matching orders.