How well (fast) does this work with out having an ac adapter?

An AC Adapter?? Perhaps you placed your question on the wrong item. The item in question is a storage box to place a standard (external) 3.5 inch hard drive into. It does not charge anything, it does not power anything, it does not use any power. It simply gives you a safe box to place a hard drive when it is completely disconnected and removed from your computer.