Very long time to open/ show a specific PDF file

Distribution

Linux Mint 21.3 Cinnamon

Package version

4.0.2

Frequency

Always

Bug description

A specific PDF file (attached) needs minutes to be shown - is seems to be (very) complex to render.
XReader uses only ONE of EIGHT available Threads of my Intel i7-4702MQ CPU @8GB RAM (Fujitsu Lifebook E744) - so the time could be much shorter using all available resources.
User without looking at CPU load could think XReader hung up.

Steps to reproduce

Just open the attached PDF "Killer_PDF_toom-Anleitung-PDF.pdf"

Expected behavior

Better performance from End-user-point-of-view.
Usage of all free HW resources
Visual information about opening process (progress bar) to show XReader is busy

Benchmark Testing

XReader (4.0.2) needs 150s to open the file, all pages immediately accessible

Brave (142.1.84.141) needs 90s to show first page, but additional time to show new page (even it it was shown already before)

Okular (Flatpak 25.08.2) needs 170s to open the file completely. Before that new pages needs 20s to be shown properly

Firefox (145.0) needs less than 15s, all pages immediately accessible.

Master PDF Editor (5.9.94) needs 15s to open the file, but additional 2s per new page.

Xournal (Flatpak 1.2.8) needs 70s to open the file, all pages immediately accessible.

Firefox performs by far the best.

Killer_PDF_toom-Anleitung-PDF.pdf

Additional information

It seems none of the PDF viewers mentioned above uses more than one CPU thread - so here is potentially big room for improvement.

Start by reproducing the delay with the attached Killer_PDF_toom-Anleitung-PDF.pdf on Linux Mint 21.3 using XReader 4.0.2, then compare its opening time with the reported viewers. The issue mentions rendering, CPU usage, and missing progress feedback but names no source files or tests; done would require a measurable improvement or clear busy-state feedback.