阻止用户从Web应用程序复制和粘贴文本的最佳方法是什么？

The site I'm working on displays some proprietary 3rd party data that's quite valuable. As such they want to stop people copying and pasting their information. They understand that, of course, there's nothing we can do to stop users just writing down info or printing it off, but they want to make it as difficult as possible for their data to be taken. The other big concern is performance. The site sees a healthy amount of activity, so keeping it snappy is a big deal.The site I'm working on displays some proprieta