It is slower than all readers. When the progress bar is pulled, the images in the PDF will be displayed with a delay. the video can see that :https://youtu.be/cDRulbJ9V1A
The reader in the Zotero 7 beta is significantly faster than Zotero 6, so you can try with that. It's not going to be as fast as Preview — nothing is — but it should be equivalent to viewing PDFs in Firefox.
The pdf document I used for testing is a paper with only 16 pages!!!
It would be more about whether there are many or large compressed images. I'm not sure what's going on with that figure that renders slowly, though — that may be very complex for some reason.
If you're finding something slower than you would expect in Zotero 7, link to the PDF or send it to support@zotero.org with a link to this thread.
If you're finding something slower than you would expect in Zotero 7, link to the PDF or send it to support@zotero.org with a link to this thread.