Why NAND FLASH is slower than flip-flops?
First: Be careful not mixing up NAND gates (which are logical circuits) and the gate contact of a MOS transistor. These are two completely different things.

The idea having NAND gates makes the internal circuit identical is wrong. Flipflops inside a chip are usually not made from gates but simplified to save space. See e.g. an SRAM cell made from CMOS pass transistor logic.

But indeed, the reason why writing to Flash memory is substantially slower is because of the large amount of charge which has to be put through an isolator (SiO2, so the drift velocity is much lower) onto the floating gate. That takes time, while the transistors which have a normal gate contact can have that one fully charged and de-charged within picoseconds.