It could even be that lots of queries to generate pages just fetch private posts and comments. The assumption was that users will not be subscribed to lots of subreddits that they have access to so skipping these posts is typically negligible and not worth using a more complex query for. If the fraction of skipped items becomes significant these queries will be doing lots more work. Its pretty believable that some very common query getting 25-50% more expensive will be more server capacity then Reddit has available.
Another possible explanation is that operations on private subreddits are not as optimized as it is expected that they have a small number of subscribers and take up a small amount of total resources. These assumptions would be flipped.
Another possible explanation is that operations on private subreddits are not as optimized as it is expected that they have a small number of subscribers and take up a small amount of total resources. These assumptions would be flipped.