Fluctuating performance with NVME SN730

Hello everyone,
I received my new Lenovo Yoga slim 15ITL05 with SSD NS730 1To I did some tets and I noticed a fluctuation in the writing speed for no obvious reason. It can go from 3100 MB/S gbs to 1800 Mbs for no apparent reason…It’s not a heat issue (throtteling) because after a fresh start it behaves the same way :

I don’t know what to think, if it’s a SSD failure or something else…
Thanks for your help

And some more pictures

What’s weird, it never goes below around 1800Mbs (maybe 1 time at 1200) when it does that. Then it goes back to around 3000…

I think I have a clue : I installed dashboard software on my PC and in the performance tab I disabled “write caching window”, as soon as I deactivate it, I go from 1800 to 2800 Mbps.
When I enable it again, writing performance fall back to around 1800
Does it make sense ?
Thank you !

Depending of the test … it can be a cache problem.
Some NVMe drives write with full speed until the cache is full, after which the speed is dropping.