Box API: the speed of upload file

I'm working on an application that can upload files.Now, I can upload files, but the speed is too slow.

For example, I try to upload a file on the website, the speed is 4MB/s, but when I upload the same file with my application, the speed is 360KB/s.

I'm using c++ and the libcurl to upload files. 

Anyone can tell me what's the matter?

Thanks!

codes

Json::Value parentVal;
	parentVal["id"] = "0";
	Json::Value attributeVal;
	attributeVal["name"] = strFileName.c_str();
	attributeVal["parent"] = parentVal;

	Json::FastWriter fast_write;
	string strAttribute = fast_write.write(attributeVal);

	curl_formadd(&m_formPost, &m_formPostLast,
		CURLFORM_COPYNAME, "attributes", 
		CURLFORM_COPYCONTENTS, strAttribute.c_str(),
		CURLFORM_END);

	strFileName = strFileName.substr(0, strFileName.find("."));

	curl_formadd(&m_formPost, &m_formPostLast,
		CURLFORM_COPYNAME, "file",
		CURLFORM_FILE, strLocalFile.c_str(),
		CURLFORM_END);

	curl_formadd(&m_formPost,
		&m_formPostLast,
		CURLFORM_COPYNAME, "submit",
		CURLFORM_COPYCONTENTS, "send",
		CURLFORM_END);

        curl_easy_setopt(m_handle, CURLOPT_POST, 1L);

	curl_easy_setopt(m_handle, CURLOPT_URL, strUrl.c_str());

	curl_easy_setopt(m_handle, CURLOPT_HTTPHEADER, m_headerList);
	curl_easy_setopt(m_handle, CURLOPT_HTTPPOST, m_formPost);

	curl_easy_setopt(m_handle, CURLOPT_INFILESIZE_LARGE,
		(curl_off_t)getFileSize(strLocalFile.c_str()));