Heavy CPU load and very long runtime when task page_thumbnails for a *.7z with many (>1000) images

[LFsWang]

Heavy CPU load and very long runtime when task page_thumbnails for a *.7z with many (>1000) images

[Difegue]

The page thumbnail mechanism has been reworked to run sequentially on a single job as of the current version in order to avoid spamming all cores and filling the runner with 1 job per page when a lot of pages need to be thumbnailed.
I'd have to reintroduce some small multithreading within the single job to improve this.

However, there is caching in place, so even if the job is halted or restarted, it'll pick back up wherever it stopped creating thumbnails.

[Ayazk726]

If you have lots of images (>2000),you need about 2s to create a thumbnail.