问题原来很简单. save_path不存在，因此只要不必刷新缓存就可以下载libtorrent库，但是一旦尝试写入文件，它就会失败并且无法继续下载，因此速度变慢并最终停止.一旦添加了现有路径，它就可以正常工作.

The problem turned out to be trivial. The save_path did not exist, thus the libtorrent library downloaded as long as it did not have to flush the cache, but once it attempted to write the file, it failed and could not continue downloading, therefore the slowdown and eventual halt. Once an existing path was added it worked fine.