There's something extremely wrong with your implementation as it just takes too much to render every page.
I've done plenty of work in the past with both canvas and pdf.js (which is what you're using) and it shouldn't be that slow, at all. Perhaps you have a rogue loop that's calling a very expensive function on each pixel of every page, maybe?
Who knows, but for sure performance on that could be near real-time.
https://codepen.io/almosnow/pen/abEXBZP?editors=0011
(at the end of the blur pass it prints the elapsed time to the console)
You're right, it does get kind of slow at 2x, but not that slow, on my laptop it takes around 1 sec/page, while on your site takes 20-30 secs/page. Also, my very naive code does not take into account "warming up" and some other code optimizations to make the blur much faster, you could easily get it down to 100ms/page, I'm sure!
Best luck!
It feels like this is the sort of tool one needs (very) infrequently, and those cases don’t seem like the sort of thing where seconds really matter. I think it’s plenty good enough.
I prefer to focus on how grateful I am that the author has made this and published it for free.
When one first opens the site and nothing happens for 30 secs. you assume that the pdf you're looking at is the actual result (that happened to me, at least), then the other one pops up and you're like ... ooooh I get it!
Most users wouldn't be as patient and just leave.