Thank you @Egor Chistyakov for the help, I've sent email with a work file I usually do, tho I just put half but thats all I could afford in second try (first time saving was crashed).

Your suggestion really helpful but I had a case when I need to update my work and reopen the file with linked tif, the linked just gone without warning as usual to relink. So I had to prevent this thing to ever happen again in minimal.
And anyway, working with embedded or rasterized design more fine with me because I need to drag a lot and fast enough to layouting. Even though it is large in size but more stable and efficient for me.

For the PDF problem -- yeah I see it is default setting in export, so that's why Exporting is slower than save as. And I see, when I compare results of 'saved as' and 'exported' files, 'exported' file had double the size of 'saved as' file (IDK why it is, I did export with custom preset of my usual and same with save as but why the result different?)
Yeah and again, it is faster doing save as rather than export, but thanks again for the insight.

Working in 1:10 scale is a common workaround, and the Large Canvas mode I proposed to try basically does the same thing, but internally.
The current maximum canvas size is 2¹4 points, exactly 16384 pt, approximately 227 in / 5766 mm. Large canvas is 10 times larger.
There are many downsides to it, unfortunately (reported here at UV too), so by 'try' I mean in, it won’t be a bulletproof solution.

Also, embedded images are slower to save in general, linked images should operate faster.
Unchecking 'Create PDF Compatible file' when saving decreases file size and save time significantly too (but it would mean no placing into Ai, Ps, InD, etc.)
But treat it all just a generic advice... In my practice, I’d move each design into a separate file... and would assemble them as links onto a single canvas as scaled copies, if I ever need an assembly.

What you create is LARGE, especially in 1:1 scale, and a ton of textures!
So yes, again, please Package the file with the textures included and send it to the team (sharewithai@adobe.com. It won’t instantly solve the performance problem, but in the long run it would help them to test and fine-tune the app to be able to run better with a file like yours.

The configuration is Intel Core i7-12700F 2.1 GHz, 32 GB RAM, Nvidia GeForce RTX 3060, Windows 11 Pro — thanks.

As for the PDF problem — it IS intentional.
When you save a PDF with the 'Preserve Illustrator Editing Capabilities' enabled (the [Illustrator Default], [High Quality Print], [Press Quality] presets have it enabled by default), the full copy of your original document gets written into the resulting file, as hidden PGF block, alongside with the PDF stream. So, when you try to read the file back, it would be 'opened' rather than 'imported' (this is what would happen if you uncheck the option). This way Ai keeps the original layout in full, to maintain the editability — and this is what you see. Remember, artboards in Ai are not pages, they can nest, overlap, etc. — and is used a lot in map making and other design tasks. So if you want a PDF to be EXPORTED — uncheck the option... but be prepared the file would be harder to read back in the very same state.

There’s a separate request for this, but there’s no easy solution to sit on both chairs at once:
http://illustrator.uservoice.com/forums/333657/suggestions/32252662
Please upvote!

@Egor Chistyakov

Hi, yeah in my case if those are all still vectors or embedd/links with so much duplicate would eventually slow the program and saving progress (sometimes blank if I made many sizes apparel) so I had to raster it.

There was a case I made just a design cloth but with embedded hi-res asset. One design, 3 size, for prints, so I had to layout every size in half dozen to fit at 150-180cm width artboard (height varies).
So, have I tried working in large canvas? No, I would not even call it medium canvas while I cant even make 4 150cm artboards in a row.

one more thing, saving in PDF also not working properly if I intent to save a single page while working with many artboards. The exported file would include everything tho.

See the image, I can only do that if the design does not have motifs/texture/pattern.
And it just designing, i had to make few sizes and layout too.

I don’t think I got the part where you say about rasterizing the contents. Do you mean it happens for you without you wanting it, and that you have to rasterize to continue working? How large are these apparel designs? Have you tried working in Large Canvas mode? (https://helpx.adobe.com/illustrator/desktop/add-and-import-files/start-a-new-file/create-files-on-large-canvases.html)

The team tries to study different workflows and adapt Illustrator to real-world cases.
If you can, please share this particular file with the team — send it over to sharewithai@adobe.com (and mention the link to this very request, http://illustrator.uservoice.com/forums/333657/suggestions/50719064).
What are your full OS and hardware specs (including GPU)?

There is an ongoing work to make Ai multithreaded, step by step, component by component, and you can follow the news here: http://illustrator.uservoice.com/forums/333657/suggestions/20423296