That is "normal". It's slow because redrawing the text thousands of times is slow.
As you paste the long line in, it's constantly update the display (because of how vim deals with text, or how the terminal is handing vim text, I guess).
I tried pasting the text in vim (using iTerm) and it has the same issue, it takes a while to paste. I tried :set paste
and :set nowrap
and still as slow. Pasting the line straight into a terminal is equally slow
With the dpaste link you mention, there is a plain-text link, which you could just wget and edit:
curl http://dpaste.com/115362/plain/ | vim -