Am getting slow write speed in my Dell 7577 though it is NVMe SSD...
Is it because of this ?
There are various levels of performance in NVMe drives - they are not all the same. Your Toshiba drive is known for being one of the slower models out there -- you're never going to see barnstorming performance from it.
but you can check in the link other user get almost 1000MB/s write speed having same ssd Toshiba kxg50znv256g http://forum.notebookreview.com/threads/dell-inspirion-7577.808514/page-24#post-10620160