It's fine for short documents, but not very pleasant for long math papers. PDF.js takes ~1 second to render a single page in my thesis. In Document Viewer (evince), the delay to render a page is barely perceptible (so I would guess < 0.1 seconds), and Adobe Reader renders pages instantaneously.
I wish it only took a second to render some of the things I've opened. I've found a lot of the manuals from Automation Direct are very slow to open and navigate. For most things I ignore pdf.js and save files to desktop so I can open them with a "proper" viewer.
