So, back to the example, let's assume 10 enemies (for simplicity's sake).
You hit 7 (rounding down), but all 10 are slowed (save ends).
At the end of their turns, half of the enemies save. Of the 7 that were hit, 4 drop unconscious and 3 shake off the effect. Of the 3 that were missed, 1 is still slowed.
On their next end of turn (where 5 of the original 10 acted normally, 4 never acted and 1 was slowed), 2 are still unconscious (the other 2 wake up prone, but have no actions to stand up), and the last slowed one is free.
On their third round, the 2 that woke up must spend an action to stand up, 1 wakes up prone and 1 is still unconscious.
On the fourth round of combat, the last sleeping enemy wakes up at the end of the round.
On the fifth round of combat, the last enemy stands up from prone and acts.
And this is just going by averages, with only the most basic optimization (i.e., decent Int score).