why do slower characters often go first?

the battle literally started with palpatine using his AE stun stunning my entire team, then due to.... bad luck? i simply did not get to take any actions at all, yes his palpatine had his zeta which sped up the team after the fact, but why did he get to go initially is my question, normally against this team i halt thrawn then stun palpatine with kylo and kill him between kenobi taunts and he rarely gets his ae off, its a battle ive done successfully 9 out of 10 times.