Robotdigg linear guide not smooth motion
Difficult to say with the given data, but here are a couple of ideas:
First theory:

The belt is too thin/flexible for the linear guide.

Linear guides remove the backslash by adding quite some pressure in the ball system: The block is slightly too small (compared to the rail) and thus it compress the balls and rail. The backslash is avoided until you reach this force.
This pressure require a little bit of force to move the block along the rail, so if your belt is too thin or flexible, the belt will start to move until the elasticity of the belt reach the force required to move the block, then the block move a little bit and stop again.
In a fast movement, your block never get to stop, while in a slow movement, it has time to start and stop. Maybe what you feel like &quot;not a smooth movement&quot;
If you identify this case, I suggest you the following solutions:

Lubricate correctly the guide, this could be enough to reduce the issue to an acceptable level.
Change the belt to a larger one, with fibre glass inside (not only rubber).
If I am correct, the &quot;H&quot; of your MGN12H stand for the pressure of the block. H is for high pressure, switching to a C is far enough for a 3D printer and will reduce the drag.

Second theory: (Thanks to FernandoBaltazar)

You got some dust, rust inside

Remove the belt and move the block with the hand, if you are able to feel the movement is not smooth, then it's probably this case.

Add some grease (Never in your life use cooking oil)
Clean the dust of the rail and exterior of the block
Move the block from one side to the other of the rail until the movement become smooth.