Fees aside, what factors could account for performance differences between U.S. large-cap index ETFs?
The "ideal world" index fund of any asset class is a perfect percentage holding of all underlying assets with immediate rebalancing that aligns to every change in the index weighting while trading in a fully liquid market with zero transaction costs. One finance text book that describes this is Introduction to Finance: Markets, Investments, and Financial Management, see chapter 11.  Practically, the transaction costs and liquidity make this unworkable. There are several deviations between what the "ideal world algorithm" ("the algorithm") says you should do and what is actually done. Each of these items addresses a real-world solution to various costs of managing a passive index fund. (And they are good solutions.) However, any deviation from the ideal index fund will have a risk. An investor evaluating their choices is left to pick the lowest fees with the least deviation from the ideal index fund. (It is customary to ignore whether the results are in excess or deficit to the ideal). So your formula is: This is also described in the above book.