Does long-polling have performance issue

Question

I'm going to use long-polling in my asp.net mvc application. Some of the connections will probably be pending for response for more than 10 minutes. I know the connections will be "hung up" so they won't occupy the connections in IIS connection pool for a long time(correct me if I'm wrong). But with the amount of user grows, the concurrent connections will be more and more, the web server will have more "background" threads. Will it cause performance issue? If yes, what's the approximate order of magnitude that the server can handle? And what other better solutions should I take? Thanks.