As someone who makes lots of merge requests, please don’t add the further statement. In my experience while it’s well intentioned, it (understandably) frustrates contributors because it’s making an assumption that they aren’t valuing your time, which you don’t know.
Just my 2¢, at the end of the day do what feels best for you, but you may drive away some contributors.
I think that statement should only come after something that justifies it.
The statement implies that the requester is acting in bad faith. If they are, fine, but if they aren't it's somewhat bad faith to treat them as if they are, and in my experience it can really change the tone of a community when all initial interactions start out assuming bad faith.
As someone who has to deal with a lot of PRs, yes I'm going to tell you this if you're being pushy. Of course I won't say this if you're polite like most people are.
Just my 2¢, at the end of the day do what feels best for you, but you may drive away some contributors.