NAND can be done by having transistors in series.
so the overall resistance will be greater compared to NOR with parallel transistors.
the delay is directly proportional to R.
the NAND will have more delay.
NOR gate only .This can be viewed from the circuit itself . You have a series connection of two PMOS of greater W/L (4 times that of nmos to have same rise and fall time ) . so this will increase the charging time in case ogf NOR gate compared to NAND gate .
When you say the NMOS and PMOS, you are refering to the peripheral circuits around the flash core array (eg sense amplifiers) but not the flash core array itself.