Hi - I am seeing a simliar slow down with my TS-673A mounting shares over SMB on a macOS Studio running Sonoma. I’m using Blackmagic Disk Speed Test set to 1G files. I have an NVMe read/write cache. After updating to 5.2.5.3145 my write speed is 18MB/s and read is 189MB/s. I downgraded back to the OS the NAS shipped with, 5.1.9.2954 and my write speed is 226MB/s and read is 270MB/s.
Seems like something really bad happened to the 5.2.5.3145 release.