It's a good idea to check for plagiarism before posting anything here, but if you come up with your own ideas, there's no need to do it again because you can be sure that what you wrote is original. If you do, it will still show you that there's no plagiarism, in my opinion. The only time I check for plagiarism, though, is when I want to start a new thread because I can see that there are already several threads here.
How long will it take you to respond to a post if you have to checked it for plagiarism? These will probably take you a while. All I can suggest is that you should try to understand what you want to respond to in order to develop your own viewpoint on the subject. When you wish to respond to a post, I don't think you need to check for plagiarism for reply. Just be careful not to paste someone else's concept and claim it as your own.