I was trying to render a scene which was working well at decent speeds(30 sec per frame) and the mem and peak were around 3000m …but i restarted the render without making any changes and speed went down to 2 minutes per frame and me and peak were around 2000m…i tried everything but nothing is getting the speed back to what it was…
rendering on laptop with Nvidia rtx3060 in Cycles(optix) blender 4.3
Welcome!
Did the change happen after modifying the scene? Does it happen on specific frames of an animation? Does the render slow down mostly because each sample is slow, because the build times went way up, or both?
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It’s hard to say what might be happening without seeing the scene.
I may have a guess, the smallest change I know that can cause that big of a slowdown without you realizing and without increasing the memory consumption.
Do you have an object with many long, narrow polygons? Was that object suddenly rotated in a diagonal? Long, diagonal faces mess with Cycles’ optimization system. If you have such an object, add loop cuts to split the faces shorter.
though I don’t know why the memory would go down.
Temperature might be your problem. Notebook easily reaches TJ max and a throttling happens even if it is just for a millisecond. After that overall performance decreases drastically and a reboot is necessary. I had a notebook and I disabled CPU turbo feature and also removed battery. If the problem was your scene, it would happen in the first frame. Start to monitor your temperatures. GPU also has individual throttling, and can be controlled by undervoltage but I don’t recommend it.
Do you use simulations? 
well thank you guys I tried upgrading the gpu and cooling it down and other optimization settings with gpu …it improved though not same as before but it’s fine…sometimes it’s fast sometimes slow
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