When you create a spawner or change the EntityId by right clicking a spawner with a spawn egg (even MC-80629 works), the spawner uses the Delay tag to determine the rotation speed of the entity. That make not much sense because the Delay tag only increases until it reaches 0.

The animation speed should be rather determined by
max((MaxSpawnDelay-MinSpawnDelay)/2,MaxSpawnDelay)

else you can continously right click a spawner with a spawn egg and it will change the rotation speed always depending on the Delay value (low Delay value = fast, high value = slow).