How much secure is App_Data Concering XML files?

I had been making an Admin Page and I dont want to use sql database to store Username / Password
I have 2 options , 1. to hardcode password + username
2. to store username and password in XML file in App_Data , and I'ad been wondering how secure it is..

p.s , XML vs SQL , XML is faster right , I'm not wrong ? :D?

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