Its not a performance optimization, but its categorized as one.  The reason being that they didn't want to completely break every existing Git implementation from being able to read a newer style index.  I don't claim to understand what they did, I'm just saying what it is.  The "standards" say any extension named with 4 capitals is an optional extension that can be safely skipped.  All other extensions must be rejected without further processing.  'REUC' is in the former category.