I'm not aware that either is faster, but if it is, it's likely such a small difference as to be lost in all the other random factors that could affect the response time. While it is true that with "." the parts are first concatenated and then output by echo, on the other hand with "," each argument is processed separately by echo, so you may in effect be executing a separate echo on each, which might be less efficient. Or not. Like I said, it's the sort of thing I don't worry about when modern processors are so fast. Maybe if I had Facebook type traffic I'd do some benchmarking and find out. (But then, they're compiling their PHP into executables, so they probably don't worry about it, either.)
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation