Illustrator suddenly switching to CPU preview, GPU preview greyed out
My little research has shown that the preview mode changes when zooming if the cmyk document contains cmyk images. In a cmyk document with rgb images or an rgb document with rgb/cmyk images, there is no gpu to cpu preview switching when zooming. To reproduce the bug, the document must include multiple images in order to impact performance during scaling. My system: i5 8400, 32 Gb, GTX1650 with latest Studio Driver installed, Asus PA329C, windows 11 Build 22621.1265, Illustrator 27.3.1 Here's what it looks like on my system.

It is expected to have this bug fixed in the latest version, 27.4. Please test if it works fine for you and comment back.
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Marketing Stockprint commented
Still having this issue.
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Sergey commented
With Ilustrator update 27.4, I no longer see this bug. Thank you!
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Sergey commented
Hi all! Looks like update 27.4 fixed the issue.
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Roy commented
I'm here to support this post - newest versions of Illustrator have a very bad bug with files switching to CPU mode and not being able to switch back to using GPU.