SDSSDP-064G slow write speed...

Hello, i have problem with my disk. It has about 6 months. works but write speed is very slow around 5-15mb/s. And in SanDisk ssd toolkit dont show a serial number.

strange it is not showing a serial number or the ATA version. where did you get the SSD? I would try a secure erase and a full reinstall see if that corrects the issue. If not contact sandisk support.