Here is an open access example:
Sneha Ravi, A., & Dalvi, S. (2024). Liquid Marbles and Drops on Superhydrophobic Surfaces: Interfacial Aspects and Dynamics of Formation: A Review. ACS Omega, 9(11), 12307–12330. https://doi.org/10.1021/acsomega.3c07657

It seems to be related to the new figures and references extraction feature.
This is consistent with the large number of linked figures (16) and references (238) in the paper above.

It looks really nice!
https://s3.amazonaws.com/zotero.org/images/forums/u265723/86lx9ergy6ec8lp55zb1.png

But it becomes a lot slower to load.
It seems to load the pages by parts:
During loading:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/1ndtrdipjbitpsvniqkn.png
After loading:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/21lc6vs5ax2cgatftti0.png

Another open access example that is really slow:
Rosselló, J. M., Hoeppe, H. P., Koch, M., Lechner, C., Osterhoff, M., Vassholz, M., Hagemann, J., Möller, J., Scholz, M., Boesenberg, U., Hallmann, J., Kim, C., Zozulya, A., Lu, W., Shayduk, R., Madsen, A., Salditt, T., & Mettin, R. (2024). Jetting bubbles observed by x-ray holography at a free-electron laser: Internal structure and the effect of non-axisymmetric boundary conditions. Experiments in Fluids, 65(2), 20. https://doi.org/10.1007/s00348-023-03759-9

There are many steps that can be slow or unresponsive during the loading, so I will try to narrow down simple steps to reproduce.

In the latest beta: 7.0.0-beta.83+066eda731 (64-bit).
I open this file in Troubleshooting Mode with only one tab opened: https://doi.org/10.1007/s00348-023-03759-9
I let it open for 5 minutes to make sure that there is nothing still loading or running and disable the automatic syncing.
I click and hold on the scrollbar and make it move down to the end of the paper and up to the top of the paper again 10 times quickly before releasing the scrollbar.

Then it stays in this state for more than 3 minutes:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/z7aoxvfcgsdb86vd34u4.png
https://s3.amazonaws.com/zotero.org/images/forums/u265723/bd7dfcyeqmcd2wzkm421.png

Do the same in PDF.js in Firefox, it recovers after only 7 seconds.

Do the same in Adobe Acrobat Pro, it works perfectly: the content of the PDF file is always visible while scrolling down and up. Acrobat never hangs while displaying the content of the PDF file.

@martynas_b: And here is an illustration of these two problems:

Just after the text appears in the PDF Reader, it cannot be selected

In some cases, the right-click release to finish the text selection is not captured

The screen recording shows all the clicks so that you can see the problems.

You can also see a case where the text selection was successful, the highlight popup window appears briefly, and disappears (0:41).

I do not see these problems in PDF.js in Firefox: text selection is working immediately after loading.

It is much slower in Zotero when doing many loadings in a row.

Do you need some more examples than this one?
https://forums.zotero.org/discussion/comment/464361/#Comment_464361

I have tested on Ubuntu 24.04 LTS on a separate recent powerful workstation with the same library.
For the simple test case I have described above, I get a score of 2 minutes 30 seconds on Ubuntu... so not much better than on Windows. Have you tried reproducing the detailed steps I have described (not just a simple loading)?

Maybe a screen recording will help to illustrate the comparison? You can see the comparison with PDF.js in Firefox from 1:36.

I have also made the test in a VM on the Ubuntu 24.04 workstation:
Zotero 7.0.0-beta.83+066eda731 (64-bit)
Ubuntu 24.04 LTS
Account containing only 3 items including the test case 10.1007/s00348-023-03759-9. Not synchronised.

I got a score of 2 minutes 15 seconds on that test to load the text after moving down and up 10 times.

Anyone else can test?

Yes, I get that error from time to time as well, but it doesn't explain the performance issue.

@martynas_b: A strange observation... It seems that the loading of the PDF files is much faster when I am not in Troubleshooting Mode...
Is there anything special about the Troubleshooting Mode that would interfere with the PDF loading process?

Thank you. Then the problem is solved.
I can still trigger the problems above in normal mode, but not to a degree that is really problematic.