TextEdit is plenty fast for me when viewing medium-size text files on a Macbook. Text files don't have "attachments" so it would seem the files you are viewing are something else. Windows Notepad *can't* open anything but text files. Are you thinking of WordPad not Notepad? What computer are you using that runs TextEdit slow?
As long as you're only viewing, pressing space bar for Quick Look in Finder works well a lot of the time.
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As long as you're only viewing, pressing space bar for Quick Look in Finder works well a lot of the time.