Strange performance on M.2 drive

[JayCool]

Hi
Newbie here.
I was transferring about 400GB of video files from a SATA SSD (KINGSTON SA400S37 960G) to an M.2 drive (Seagate BarraCuda Q5 ZP1000CV30001). Initially it achieved >1GB/s performance but after 30 secs or so it plummeted to ~70MB/s, whereas I expected the bottleneck to the the Kingston disk at ~400MB/s.
The Seagate disk is where the OS is installed and the PC became unresponsive.
Is this normal behaviour? Is there a cache or buffer that gets filled on the M.2 disk?
I attach an annotated screenshot of MS performance monitor.

Thanks for any advice/help you might be able to give.

[David (PassMark)]

I've also seen a few strange things with M2 SSDs recently.
I think there are a bunch factors that all get mixed up.

1. Caching on the drives and in main RAM (by the OS) means you don't get a constant copy speed. It can run in fast bursts, then a slow period, as the cache fills and empties.
2. Some SSD just don't give consistent performance. Especially the cheap & mid range units. I saw speeds dropping to 30MB/sec on a Kingston 2TB M2 SSD the other day. Samsung SSD did 500MB/sec on the same files in the same machine.
3. There are still a lot of device driver and physical interface issues going on with M2 drives (e.g. using SATA interfacing in a M2 slot). External thunderbolt and USB drives are even worse for this.
4. Some drives slow down when full
5. Some drive slow down when hot
6. Some drives (or maybe drivers) don't seem to deal with concurrent activity very well. (i.e. background disk activity hurts)
7. Sometimes you might be seeing a forced TRIM operation interrupting normal activity on drives in heavy use.

It is a bit of a mess.

[JayCool]

Wow! Thank you for the detailed response.
Samsung have decided to replace the drive, so I am hoping it was faulty. Failing that I shall go through your list to see whether I can tack down the cause.
If I resolve it, I'll share.

[David (PassMark)]

From a SATA SSD (KINGSTON SA400S37 960G) to an M.2 drive (Seagate BarraCuda)

Samsung have decided to replace the drive

That was good of them, considering it a copy issue from a Kingston to a Seagate