Progress tracking an NSURLSessionDownloadTask using KVO

I have a piece of working code that downloads a file using NSURLDownloadTask, that tracks the download progress using URLSession:downloadTask:didWriteData:totalBytesWritten:totalBytesExpectedToWrite:. The progress indicator fill in smoothly - it looks good. I am now refactoring the code to not use session and task delegate methods for various reasons. For progress indication I switched to KVO on the download taks's countOfBytesReceived (the documentation notes that all task properties support key-value observing). It does work, but it looks pretty bad - the intervals between progress incrementation take too long and sometimes never show up at all. To my understanding, I should not explicilty dispatch the task to a background queue - it should be done by the session task itself (or should I?).

Example code follows (please excuse the vintage ObjC syntax...)

- (void)download:(NSURL*)url {
        self.task = [session downloadTaskWithURL:url completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            [self stopObservingProgressChanges];
            // ...
        }];
        [self startObservingProgressChanges];
        [self.task resume];
}


- (void)startObservingProgressChanges {
    __weak WTPackageDownloader *weakSelf = self;
    [self.task addObserver:self
                   keyPath:@"countOfBytesReceived"
                   options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew
                     block:^(MAKVONotification *notification) {
                         weakSelf.package.progress.totalUnitCount = weakSelf.task.countOfBytesExpectedToReceive;
                         weakSelf.package.progress.completedUnitCount = weakSelf.task.countOfBytesReceived;
                     }];
}


- (void)stopObservingProgressChanges {
    [self.task removeObserver:self keyPath:@"countOfBytesReceived"];
}