How slowly can you clock an AT25DF641A SPI Flash?
The way to check this is in the timing diagram which is in most datasheets with communications specifications. There are no maximum times for \$T_{CLKH}\$ or \$T_{CLKL}\$ which means the SCK can be as long as you want. There are also no maximum restrictions on CS times either so that can be held low or high for as long as you want.

Source: https://www.mouser.com/datasheet/2/590/doc8693-1385727.pdf