Batch updating CoreData without performance issues

Question

I have a tableView which I populate with cells. The data is being fetched as json from a private api and stored as CoreData. As the cells are being reused I fetched their imageView via a delegate method (from web URL). The problem occurs when I try to save that image data into that managed object. So when the image data is fetched, I fetch the managedObject, add the image data and re-save it. However this has a tremendous impact on the tableview performance (lagging tableview scrolling). I tried fetching and saving in background (privateQueueConcurrencyType) but nothing changes. Any recommendation is greatly appreciated.

func updateCoreDataImage(image:Data, for Url:String) {
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let context = appDelegate.persistentContainer.viewContext

    let priMOC = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
    priMOC.parent = context

    priMOC.perform {
        do{
            let request = NSFetchRequest<NSFetchRequestResult>(entityName: "DB")
            request.predicate = NSPredicate(format: "url = %@", Url)
            request.includesSubentities = false
            request.returnsObjectsAsFaults = false

            let result = try? context.fetch(request)

            let object = result![0] as! NSManagedObject

            object.setValue(image, forKey: "image")

            try priMOC.save()
            print("save")
        } catch {
            print(error.localizedDescription)
        }
    }
}