dlloyd:
If I am using a lot pins to read value with a processor that runs at 16 MHz which is on MEGA2560 which requires a lot cycles; so, it makes MEGA2560 very slow.
Note that as the number of inputs is reduced ... this increases the outputs required for address lines and the addressing/decoding/formatting/conversion of signals will progressively get more complex and slower. Also note that the total number of reads required would not change, no matter what approach you take.
I know that before I started the topic in Arduino forum.