when the load is low, collisions will be less and there are more chances of a frame to be transmitted successfully. in pure ALOHA a station will transmit a frame whenever it is ready
But in case of slotted ALOHA even if the load is low a station has to wait for the start of next time slot to transmit its frame(even if the frame is ready)
Hence, delay in slotted ALOHA will be more(stations have to wait for the next time slot)