Slerp is an interpolation. The final parameter T must be a ratio of TimeSinceLookingAtLastTarget/TotalTimeDesiredToLookAtNextTarget.
0 <= T <= 1
If you expect it to take 2 seconds to turn from target A to target B, and it's been .25 seconds, the interpolation ratio should be 0.125.
Time.DeltaTime is just the number of seconds in took to process the last frame. It should always be consistently low and won't change much. At 60FPS, that code would leave you looking 1/60th of the way between two targets.
So you will need to know the actual time and how long it should take. When you start the turn, capture
float SecondsToTurn = 2; //adjust this
float TurnStartTime = Time.Time;
Each frame calculate your slerpT as
float slerpT = (Time.Time - TurnStartTime) / SecondsToTurn
Use slerpT instead of Time.deltaTime in your Quaternion.Slerp