the EPC keys are in HEX
I don't think looping through 75 records is going to cause a noticeable delay from a user perspective. Try it with your simple version and see before you try to optimize.Obviously, getting more capable hardware will let you store the keys in RAM, but again, prove to yourself that you need to do it.
A alternative method of this to create individual text files named for each valid tags last 9 digits. Inside each file have the full ID that is valid, if for some reason you end up having two keys with matching last 9 digits then have two entries in the file with each full ID.Now when your sketch receives a ID code, you can create a string based on the last 9 and then try to open that string name on in your SD.If the file exists, read and compare.
I suggest there is a problem with this option. The SD uses logic from the old floppy disk days. EACH FILE will take 512 bytes of space from your SD card. If you go this way, be sure to use a large capacity SD card.Paul