Core data 由于cpu时间超过限制，coreData后台线程被终止

My watch extension使用以下代码（简称）在后台线程中从coreData获取许多项目：

显然，这需要太长时间

我的问题：

coreData后台线程有时间限制吗

如果是这样，我如何修改代码以避免它

    coreDataSerialQueue.async {
        backgroundManagedContext.performAndWait {
            …
            let buyItemFetchRequest: NSFetchRequest<CDBuyItem> = CDBuyItem.fetchRequest()
            …
            do {
                let cdShoppingItems: [CDBuyItem] = try backgroundManagedContext.fetch(buyItemFetchRequest)
                …
                return
            } catch let error as NSError {
                …
                return
            }
        }
    }

Event:            cpu usage
Action taken:     Process killed
CPU:              2 seconds cpu time over 4 seconds (59% cpu average), exceeding limit of 14% cpu over 15 seconds
CPU limit:        2s
Limit duration:   15s
CPU used:         2s
CPU duration:     4s
Duration:         3.57s
Duration Sampled: 0.00s
Steps:            1