Slow speed shuttlecock are usually used in area that are in higher elevation. It is also used by coaches to train strength of stroke and for building stamina (longer rally) to train muscles on the arms.
Yonex Mavis 350 is the official nylon shuttlecock used throughout high school teams and tournaments in the Northern California. It is also used by many European countries for their schools and junior national training.
The Mavis 350 is one of the best yellow nylon/plastic shuttlecock. It is an easier shuttle to see and play when the wall or surrounding is white. It has the closest performance to feather shuttlecock in flight and feel. Though it cannot totally mimic feather shuttlecock, it is close enough for high school and junior national to use for play, practice, and club matches or tournaments.
Shuttle Review 
Durable and good quality 
Shuttlecock is fine