The PDF Embed API seem to use a sort of lazy rendering, for example if you zoom a page and then scroll to another page you may just get a blurry upscaled version of the page and then after a time it renders the page again and it snaps back to being a sharp image, similar on load if you scroll the page can be initially blank until it renders.
Unfortunately I can't see there is any event so say that rendering is now complete. Use case for this is that users staring at a blurred or blank screen for a while tend to complain, and also specifically in our case the user is meant to be reviewing the PDF and not aware the blurred page is just temporarily while waiting to render the page.
The event would be useful so we could display some spinner to notify the user tha app is currently busy rendering.
Yes the delay was significant, especially when zoomed in. We have been through this scenario a few years back with our users, we initially thought the time delay was acceptable but you end up with a number of users with either a slow PDF or zoomed in and they generate complaints to support and we had to put up a spinner to keep them happy. At the end of the day they weren't really wrong, delays are more significant when you are focused on the document to complete a task rather than just looking at the time delay in absolute seconds 🙂