Is Deceleration spell useful?

[IanTheWizard]

I just tested a Deceleration Slow spell whose efficiency is 5 on several mobs.

• On a melee mob, his speed seemed to slow down for about 0.1 second. So it's nearly no difference.
  
• On an archer mob, I couldn't see any difference.
  
• On a spellcaster mob, her speed slowed down for only 1 second (similar to the effect of arm injury).
  

Creating a Deceleration Slow spell of 5 efficiency and 10 seconds duration already requires your Astral Magic knowledge to be at least 63 and costs at least $83,000. It would be more expensive to increase the efficiency of the spell.

Is this spell useful or not?