I don't know WHY - but I found a way to speed up some renders 10x

I don’t know WHY this works, it even works if you save over the old file.

But

If you have a render taking a wierdly long time to render, try moving everything to a new project. Simple copy/paste. You can save over the old project and with identical settings, I’m seeing a 10x+ speed boost!

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Have you checked if cleaning up unused data blocks gives you the similar results? I’m just wondering why you’d see that improvement, and wondering if the issue is old data. File → Clean Up → Recursive Unused Data-Blocks. Sometimes old data hangs around longer than expected so I like to use that to keep file sizes smaller.

I have. I’ve also cleared orphan data.

yep same happened me once when modelling with heavy models, i just opened a new blend file and paste some of them, then it goes more faster the modelling operators, and getting in and out of object and edit mode…