Going back pretty much as far as I remember, putting together speaker notes for a deck of slides (or, worse, trying to cajole someone else into putting together a set of speaker notes for their slides) was one of those perpetually incomplete tasks. It just doesn't get done. But this can be a real problem for decks designed for others to deliver--especially when the deck itself isn't especially self-documenting. It also means that "the slides" attendees want after conferences aren't, in fact, any more useful than other conference tchotchkes.
I've come across what, for me, is a very effective solution.
Regular readers will probably have noticed that I publish transcripts to go along with my podcasts. My reasoning is that a lot of folks, including yours truly, find it generally easier to skim through text than it is to find the dedicated time to listen to a podcast. I use a service called CastingWords, which is essentially a front-end for Amazon Mechanical Turk. It's both high quality and cost effective--$1.50 per minute plus or minus depending upon how quickly you want your transcript.
For speaker notes, I now just record myself "giving" a presentation--often in slightly abbreviated form. I have that recording transcribed, do some light editing, and then cut & paste into the slide notes. One could also put the slides and associated notes into a separate text document.
This approach is a lot less effort--and certainly less mental effort--than sitting down and typing up a full set of speaker notes. The notes are probably more detailed doing it this way as well.