Greetings! 
I'm facing same issue as in these topics both in web and Win 11 desktop app
https://community.miro.com/ask-the-community-45/board-stops-working-big-pdf-file-5172
https://community.miro.com/ask-the-community-45/board-optimization-with-outside-objects-7375

My pc though has i9-14900k and GTX 4090. So there is almost no load on PC when I work with miro. But miro becomes completely unresponsive when working with PDF with a lot of elements. Obviously this is optimization issue. I tried to export frame as PNG but 5894x4096 is not enough for me to see some elements. After I zoom to PDF once - whole board is unresponsive even if I zoom out. Only moving focus away from pdf helps.
Same PDFs work well in acrobat reader or browser. 
So this problem was not resolved? It's a serious issue and at least optimization guide for this topic has to be released.
My suggestion for easy fix will be to allow option to convert PDF to PNG internally in high-res above 8192x4096 limitatation. Actually even 8192x8192 would be enough in all of my cases but 4096 is a real bugger.