What makes a motor hard to spin?

[zeropointbug]

Yes, precisely.

Also, the difference between the cores of slotted and slott-less motors are:

Slotted: Has coils wound in and around iron 'slots' that run length wise the motor, which creates the defined notch when you turn the shaft.

Slot-less: Has the coils wound in and around themselves, and compared to a slotted design, I believe they use more copper. There is only an iron 'blanket' around the coil to help intensify the field when running, and to shield components around the motor from intense AC magnetic fields.