Hmm, not sure about number of words, but I notice little to no slowdown in notes from 3 to 28 megabytes, with graphics &, tables, even PDF notes get only somewhat slower.
But I have a number of notes copied from the internet, 1 megabyte or less in size, but they are copies of heavily formatted pages, one of them is from a Facebook page. Those notes are very slow to display.
In general, I find information I copy from the web with formatting preserved, may go from slightly slower to very slow, so I usually strip things up when I can, unless the formatting is important.
It may also happen if the note has errors, in such cases I rebuild the note, just in case.