使用ios8，performFetch很慢

Question

我正在使用项目的核心数据，iOS 7的以下代码比8

更快

var fetchRequest:NSFetchRequest = NSFetchRequest(entityName: "Test")
let positionDescriptor  = NSSortDescriptor(key: "position", ascending: true)
let remoteIDDescriptor  = NSSortDescriptor(key: "itemID", ascending: true)
fetchRequest.sortDescriptors = [positionDescriptor, remoteIDDescriptor]

fetchRequest.predicate  = NSPredicate(format: "item = %@", self.item!)
fetchRequest.fetchBatchSize = 20
self.frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)

self.frc?.performFetch(nil)
self.frc?.delegate = self

例如，如果我获取20个项目冻结UI 2到3秒

感谢任何想法！