> It would be really helpful if the thumbnails could be created more quickly.

this may well be a Unicorn wish. 

it may be that with all the thumbnails being requested at once it causes a bottleneck, and it may well be better to request fewer pages to be rendered at a time. that may also help with the "starting from page 1 onwards" (though really that needs to be "starting from the first thumbnail visible onwards" otherwise if you are at page 200 you'll wait quiiiite a while for the thumbnails ;)

however, this may already be as good as it can get with a managed thread pool in the okular renderer, etc. it's also hard to know how good is good enough. "more quickly" is not a measurable goal :)

You are absolutely correct. I wrote this bug report purely from a user's perspective and did not try to abstract from that and switch to a UX professional's perspective, sorry about that.

The problem we're facing here is not that something needs time, it is how the user _perceives_ it. When I open the drawer, I see a grid with mostly placeholders and a few thumbnails scattered here and there. I don't _see_ that the system is still working to create more thumbnails, it just looks broken (look at it again yourself, you'll notice it).
So if it is not possible to create the thumbnails faster, we need to convey to the user "Okay, these are the pages of your document. Normally I would present thumbnails of them to you, but I'm still busy creating them". This is not conveyed by the current UI. Currently a user thinks "Hey, why do some pages look like the real ones, but most others like placeholders?"

The actual rendering in the main view does a better job here: It starts with a blurry image of the page which is gradually sharpened as the details are rendered. Since I know that my document isn't really all blurry, I understand that the rendering is still at work.
So how long would it take to create a very blurry thumbnail (just sharp enough that not all pages look _exactly_ the same to users)? If that is quicker, than I'd suggest rendering very blurry thumbnails at first and then gradfually replacing them with sharper ones. I bet users will hardly notice what's happening.
If that would take almost as long as it takes to create the current ones, we need a different solution.

Actually, I'd say the thumbnail for the currently viewed page should be rendered first. This is not the case right now and it looks extremely weird when I see the crystal clear page before me in the main view but when I open the drawer, that same page is still a placeholder. Users don't understand that rendering the main view and rendering the thumbnail are independent processes, and it just looks totally broken.
Genereally I'd expect the thumbnails to be rendered as fast as the main view, so when I look at each page long enough to be fully rendered, its thumbnail should be there as well. Again, this is currently not the case. It takes longer to render thumbnails for X pages than it takes to render the actual pages.

So here is a measurable time for the rendering of a thumbnail: At maximum as long as it takes to render the page in the main view.

(Sorry for the very long comment. I just realized that this is actually a pretty delicate UX task.)

awesome comment; i really, really appreciate the time, though and insight offered.

i've looked at the code a bit today and have some ideas on how to hopefully improve things. will keep this report updated as that happens.

I'm glad you appreciate my long comment :) I realize that this is how I should always write about UX issues. Experiencing a product from an actual user's perspective is great for spotting issues, but when it comes to describing them to developers / designers, I should always switch to the UX professional's perspective, so that I can provide you guys with more helpful input than just "I don't like this!".

Back to the original topic: I just realized that the rendering speed of the thumbnails varies geatly between different PDFs and between different pages within a PDF. In one PDF, the thumbnails were rendered almost as quickly as I scrolled through the pages. In another one, they were redendered really really slowly. In a third one, some thumbnails were created almost instantly, whereas others took literally minutes to render.
It seems that images on a page seem to slow down the process, but even if they have the same size, some images delay the thumbnail rendering by only a few seconds, others by minutes.

Thumbnail creation feels lightning-fast now compared to before and no thumbnails are missing, great improvement!