time for i in `grep server /etc/hosts | awk '{print $1}'`; do (ssh $i "echo 123; sleep 1; echo 456" &); done

real 0m0.063s
user 0m0.016s
sys 0m0.004s

time grep server /etc/hosts | awk '{print $1}' | xargs -P 24 -I {} ssh {} "echo 123; sleep 1; echo 456"

real 0m3.747s
user 0m1.136s
sys 0m0.112s