I've managed to get around this with the current version.
It requires two things:
A mouse with a scroll wheel Dexterity I managed to add 2-3k songs at a time. You could probably do the whole thing but I didn't want to test it.
So basically, select a song and using your scroll wheel (don't drag the scrollbar down or use arrows), scroll up or down until you reach a point and then shift + click. You can then drag your songs over to your new playlist.
Looking at this, I would imagine it's due to how spotify loads tracks in your list. It only loads tracks as it sees 'em, which would explain why skipping to the end using the scrollbar only selects a handful.
By using the scrollwheel on your mouse, you're loading each song as you scroll so when you select the end of the list, it's selecting everything its loaded.
Of course i could be talking complete tosh, but as a programmer myself it kind of makes sense.