Std has the same performance as every other storage class. There are 2 async classes which you can't read from without retrieving first, but that's not a 'performance' difference as such - GETs aren't slow, they fail.
Generally speaking this isn't something Amazon S3 customers need to worry about - as others have said, S3 will automatically scale index performance over time based on load. The challenge primarily comes when customers need large bursts of requests within a namespace that hasn't had a chance to scale - that's when balancing your workload over randomized prefixes is helpful.
