With pure ALOHA transmissions can start instantly. At low load, no collisions are expected so the
transmission is likely to be successful. With slotted ALOHA, it hast to wait for the next slot. This
introduces half a slot time delay on average.
Slotted means the transmission media is time divided between all users of that media. In other words you get a while to talk and then I get a little while to talk and so on among all users. In some other forms of communications the transmitting station is considered to own the media until they relinquish that ownership. If the "low load" means only one user, or just a few users with very short transmissions, then throuput in a non slotted environment is optimum. As the number of users in a communications environment increases and the message lengths increase, a slotted transmission media becomes more desirable.
In an unslotted environment message collisions are frequent and add to the delay as load increases.