The washer is spinning, just not very fast, so the motor is okay and if the basket is turning the coupler is okay.
The most likely cause of this problem is the clutch or basket drive.
A basket drive and clutch can malfunction so that the washer will spin empty or with a small load but with a heavy load will not turn fast enough to spin the water out.
I recommend changing the clutch and basket drive for this problem.
You did not mention anything about agitation but if the washer agitates okay then the coupler is okay.
The basket should turn freely by hand in the spin direction. Clothing can be beneath the spin basket and prevent it from turning freely.
The spanner nut under the agitator that holds the spin basket to the drive block must be tight.
If you hear any squeal or squeak then the spin bearing can be the problem.
The image below contains the procedure for accessing and servicing the clutch and basket drive.
I hope this information is helpful resolving your problem but if you have any additional questions then submit another question with additional details for further assistance.