RX Java - 观察主线程的一个副作用非常慢

Question

我在我的应用程序中执行以下操作（请参阅注释以查看发生的情况，逻辑非常简单）：

// 1) import folders from system
importObservable
    // 2) emit all folders as list 
    // => 1 item is emitted only 
    // => flatMap can be used for sequential execution
    .toList()
    // 3) Log current time
    .doOnNext(data -> T.start(KEY_TIME_LOGGER))
    // 4) throw away data from import
    // load all data from database now in an empty state 
    // => 1 item is emitted only
    .flatMap(data -> loadFoldersEmpty())
    // 5) Log time
    .doOnNext(data -> L.d("Folder - 1 - empty list created: " + T.printTotalAndLap(KEY_TIME_LOGGER)))
    // 6) observe the list of folders on the main thread
    // because we will update the UI
    // => 1 emitted item only
    .observeOn(AndroidSchedulers.mainThread())
    .doOnNext(items -> onNextItemAvailable(items))
    .observeOn(Schedulers.io())
    // 7) Log time
    .doOnNext(data -> L.d("Folder - 2 - empty data delivered: " + T.printTotalAndLap(KEY_TIME_LOGGER)))
    // 8) loading full folders 
    // => this emits multiple items, folder per folder
    .flatMap(data -> loadFullFolders(data)) 
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(new Subscriber<Object>() {
            @Override
            public void onCompleted() {
                onLoadingFinished();
            }

            @Override
            public void onError(Throwable e) {
                onLoadingError(e);
            }

            @Override
            public void onNext(Object o) {
                onNextItemAvailable(o);
            }
        });

问题是，评论＃6后发生的事情很慢（约200ms到300ms）。是否有更好的方法在主线程上产生副作用？