I have an 8125, which is basically a HTC Wizard. This is arguably one of the slower WM5 devices. I can open the Notes application almost instantly. To be fair I only have a couple of dozen notes and I am not sure if this makes a difference or not. But since WM does not close an app when you tap on the X, once it is open it should stay open for you.
You don't say if you need your notes to sync with Outlook. The Notes app in WM syncs, but not categories. An option there, and this may be a better option for you anyway, is something like PhatNotes (
www.phatware.com).