As long as the commands don't produce echo outputs itself (which can be silenced - thats what the prefix is meant for), it's the same performance.
If you have doubts - run your own test, like:
Code:
alias nonsilenced {
  var %n = 1, %start = $ticks
  while (%n <= 50000) {
    var %a = $ticks
    noop
    inc %n
  }
  echo -a nonsilenced $calc($ticks - %start) ticks
}

alias silenced {
  var %n = 1, %start = $ticks
  .while (%n <= 50000) {
    .var %a = $ticks
    .noop
    .inc %n
  }
  echo -a silenced $calc($ticks - %start) ticks
}

My results (running each alias 3times):
nonsilenced 1687 ticks
nonsilenced 1703 ticks
nonsilenced 1703 ticks
silenced 1703 ticks
silenced 1703 ticks
silenced 1688 ticks
smile