This thread popped up on Google page 1 so I might just give everyone the answer that worked on my 5510 (and probably most other slow DELL NVME): https://www.rswilley.com/blog/how-to-fix-slow-write-speed-of-toshiba-ssd-on-the-dell-xps
If the site is gone, simply turn off Windows write-cache buffer flushing on that NVME drive. You're welcome!