Any updates?
Tested all drivers available with Microsoft Windows 10 1903 May 2019 Update.
Send Toshiba an e-mail, they said: read 2400 MB/s and write 350 MB/s is within de drives parameters. 
I had similar problems for over a year on my BG3 SSD in the HP Pavilion 13-AN0007NU laptop. After re-formatting it, reinstalling Windows a few times, and lastly the new firmware, the speeds have improved in seq writes. Now they range from 300 to 600MB a second, which is better than the 100MB I had before. Note that I run the Windows Dev builds.
Firmware ADHA0104: https://support.hp.com/us-en/drivers/sel…mId/ob-269278-1
CrystalDiskMark speeds still vary from time to time, but…
Test results: https://drive.google.com/file/d/1_KPapMl…iew?usp=sharing
Since this firmware disables HMB, there is a slight drop in random R/W operations, but I see much more free RAM on my computer which is a bonus. And also, 0 performance difference – no degradation.