I have looked at many questions for how to shatter ice but they all say make tiny cubes and replace the prefab with the broken one. However, when I try this, I get a lot of lag. Is there a way to make a particle effect with cubes(shards of ice)?
Having a pre made prefab works.
As long as the cubes are not instansiated, within reasonable amount of parts, you can even lookinto into physics layers to prevent parts from crashing with eachoter.
Also see Shattering Glass - Unity Answers
Video tutorial on consept Destructible Objects - Tutorial for Unity 3d and Blender - Episode 9 - YouTube
If your purpose is for an immediate use in your project, there is Exploder in the Asset Store that you might find helpful to break an ice.