The reason is pretty simple. A GoogleBot is not used to calculate the speed of a page.
Instead, Google uses Toolbar data from real users surfing and accessing your web pages with the Google Toolbar installed. Blocking GoogleBot will not block ordinary users with the Google Toolbar installed from accessing your site.
So if you are trying to hide your slow pages from Google, I'd recommend other methods, such as hiding your slow pages from your users.
Forum discussion at Google Webmaster Help.