Google Drive API - too slow.

I've just working on Google Drive API. I have one problem, it's too slow. I use methods like in the Documentation. For example:

List<File> getFilesByParrentId(String Id, Drive service) throws IOException {

    Children.List request = service.children().list(Id);
    ChildList children = request.execute();

    List<ChildReference> childList = children.getItems();
    File file;

    List<File> files = new ArrayList<File>();
    for (ChildReference child : childList) {
        file = getFilebyId(child.getId(), service);

        if (file == null) {
            continue;
        } else if (file.getMimeType().equals(FOLDER_IDENTIFIER)) {
            System.out.println(file.getTitle() + " AND "
                    + file.getMimeType());
            files.add(file);
        }
    }

    return files;
}

private File getFilebyId(String fileId, Drive service) throws IOException {
    File file = service.files().get(fileId).execute();
    if (file.getExplicitlyTrashed() == null) {
        return file;
    }
    return null;
}

QUESTION: that method works, but too slow, for about 30 second.

How can I optimize this? For example, not to get all files (Only folder, or only files). or something like that.

you can use the q parameter and some stuff like :

service.files().list().setQ(mimeType != 'application/vnd.google-apps.folder and 'Id' in parents and trashed=false").execute();

This will get you all the files that are not folder, not trashed and whose parent has the id Id. All in one request.

And BTW, the API is not slow. Your algorithm, which makes too many of request, is.