Reading from ethereum nodes doesn't take any gas but many ethereum nodes use a timeout low enough that listResolvableEvents and similar will fail as the number of markets increases. This is currently causing issues with the SportsLinkMarketFactory, at around 550 markets.
Also, there's no reason for this read-only method to be part of the market factory itself. Since market factory contract size is at a premium, it's better to expose the right read methods and move this code into the fetcher or a specialized contract.
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