Since you updated the TOS and the problem persists, it may not have much to do with TOS. SMB speed is related to many factors, and it is recommended to check other parts as well.
You can check step by step according to the transmission line: computer hard disk read and write performance > computer network card bandwidth > computer to router connection > router itself > router to TNAS connection > TNAS network card bandwidth > TNAS hard disk read and write performance.
In addition, this is also related to the files you copy and use. It is recommended to use large files for testing.