I used the AS SSD benchmark, this is the best result I got (in sequential r/w). Some people from the Internet have posted screenshots where it is possible to get writing speeds around 1000 MB/s, others have posted similar results to mine.

I also tried with CrystalDiskMark, which results in similar results.

Of course I can expect the SSD to work as fast as on other identical laptops.

I expect that the processor runs at up to 3,8 GHz. I expect that the display has the resolution of 1080p. I expect that the GTX card runs as expected.

The SSD runs at less than half of the expected speed. That is not working as designed.

Yeah, I read through that thread and did a bit of Googling around elsewhere out of curiosity.  The only thing that's clear to me is that this drive's write performance is hugely inconsistent even within the exact same system.  One post shows a benchmark that delivered the expected write performance followed by another benchmark performed only 1 minute later that shows much lower write performance like you're seeing.  If this is indeed some shortcoming of this particular SSD's design, then I don't know what if anything Dell Support can do here.  Theoretically they could exchange the SSD for a unit from another vendor, but I don't know if they would even be able to specify a particular brand replacement for a parts exchange, but even that assumes they would agree to do the exchange at all.  They may not given that Inspiron systems to my knowledge technically don't guarantee any particular performance specs with respect to SSDs the way Latitude and Precision systems do with their "Class 20/40/50" SSD designations.  I realize that's not a very satisfying answer, and it's obviously not in Dell's long-term interests to equip their systems with SSDs whose performance is so inconsistent and can drop so low, but that doesn't mean they'll have a solution for existing systems.  If I were in your position at the moment, I would probably try Dell Support once or twice to see about getting a different model replacement SSD, but if that didn't work out, I'd probably either live with it or spring for a Samsung 960 Evo.  Sorry I don't have a better answer.

Thank's for the answer. I think the 7577 supports 4 lanes, as the read speed is totally normal and matches the designed speed (around 2400 Mb/s). I guess you couldn't see it because the image I uploaded wasn't verified by the administrators yet.

The BIOS and everything is up-to-date. Only thing I also did was reinstalling Windows after I bought the PC because it was slow because of the Dell crapware preinstalled.

But as mentioned in my first comment, I'm not the only one who has this problem; the entire thread I linked to posted similar results with great read speed but low write speed.

If nobody from Dell will comment here, I will have to ask the customer support (even though I don't know if they are able to actually help...).

Thanks for your answer! Yeah, I know you can't do much about it. It's either a driver problem or just a defective drive, my guess is the driver/firmware or sth similar. I mean, the current write speed is still relatively fast, but it's just very unfortunate that you have a NVMe SSD but then can only use less than 50% of the speed.

I'll try contacting Dell support, maybe it will result in something. 

The laptop can certainly handle the drive at near its peak speeds as mine came with the 512GB version. Infact I have a 960 Pro in it now and that reaches near its peak speeds as well.

Make sure Drive settings is on AHCI and not Raid On. Make sure your drive is 4K aligned otherwise you will see such degraded performance. Also there has been a hit in storage performance with the Spectre/Meltdown patches though nowhere as slow as you are seeing. Lastly some drives don’t hit max speeds until higher Q Depths.

Worst case scenario try reinstalling windows as in without all the bloatware and without the RST drivers, use the build in MS storage drivers, even if that fails then it’s either a faulty SSD or controller.

There was another guy on notebookforums who had the same issue as you who did a fresh install and got full speeds. I always do a fresh install when I get a new laptop so I guess I never had to face this.

Thanks for your answer!

I've already reinstalled Windows after I've bought the laptop, the drive is aligned.

But I've reinstalled it with RAID on. If I would have to switch to AHCI I would again have to reinstall Windows (or is there an easy way without reinstalling?), which would be really painful. As far as I have read switching to AHCI didn't have any influence on performance?

But if there's no other way I will probably have to try it...

So, I found the way to switch from RAID to AHCI without reinstalling Windows.

The speed are still the same, maybe even a bit worse. So it doesn't help :(

I have now received a replacement Samsung SSD from Dell support, which features the normal read & write speeds. It seems like the Toshiba SSDs have some kind of problem, my guess is it's a software/driver problem and not because of faulty hardware. But just my guess.

Thanks for sharing. I experience the exact same problem. ~300 write speed. I will open a case.

Try installing Windows from clean image, with no internet connection, with no drivers, AHCI mode. Then try to test drive. Do not let Windows download any drivers.