SATA SSD running faster than SATA2 but much slower than SATA3

I recently changed the HDD to SSD. The SSD is KIOXIA TC10 which has read/write speed over 500 mbps. However, the speed accessed by Blackmagic disk speed test is Read 400mbps/ Write 200mbps.

I know some early product of A1278 has a thin SATA2 cable which may not support SATA3 speed. However, SATA2 is at most 300mbps. The current SSD can run read speed at 400mbps which is faster than the limit of SATA2. It seems like the current cable is SATA3.

I don't know if I need to replace a cable or to replace the SSD.