Hi Harukawa san,
Operating frequency range for S25FL512S flash device is between 0Hz (DC) and FSCK(max)(as mentioned in datasheet).
e.g. As per datasheet minimum frequency for single SPI commands is DC (0Hz) and maximum is 133MHz. (Table 11 on page 31 of datasheet, https://www.cypress.com/file/177971/download )
It is OK to use FSCK = 10KHz in your application, it is within the datasheet limits.
Thanks and Regards,