Traditionally, fairness in a blockchain has been defined in absolute terms, i.e. once a transaction is seen by a sufficient number of validators, it will be executed in some block, soon. Vega's proposal is to add a module to blockchains that supports the concept of relative fairness so that competing transactions may be sequenced under a known and understood protocol, and not subject to a validator’s discretion.
"If there is a time t such that all honest validators saw a before t and b after t, then a must be scheduled before b”. This is a property that can be assured of at any time with a minimal impact on performance.
To get the best combination, their current approach is a hybrid of the two. In normal operation, the protocol will assure block fairness. If the network detects that this causes a bottleneck, it temporarily switches to the timed approach (thus sacrificing a little fairness for performance), before switching back once the bottleneck is resolved. However, Vega will ultimately make the level of fairness customisable by market.