Button should look like a button (have affordance). "isn't short" is not a good label — it's not a command (you isn't short URLs?).

If button gives click feedback and you can shorten in 1-2 seconds, then you might not need a progress bar.

Explain on the page, in something that can be read in 4 seconds, what is that any why should anybody use it (problems you solve that others don't).

Assume nobody reads the blog post.