Instead of var dailyAverages:[Date:Double] I think something like var monthlyAverages: [YearMonth: [Day: Double]] would allow a more granular approach that you could pass to the views and get better performance.

The key issue is that dictionaries in Swift are value types. When you call dailyAverages.merge(), it creates an entirely new dictionary. From SwiftUI's perspective, the whole data source changed - even for dates that weren't part of this month's update.

The killer here is that Date comparisons are expensive - they involve checking year, month, day, hour, minute, second, and timezone components. When you're doing this hundreds of times per update, it adds up fast!

Can’t really answer about Swift Data but for Core Data you could do a batch of inserts using background context using something like 5-10k inserts at a time depending on the relationship between items. You could even try to do it in paralel.