Вот что пишут в интернетах, но лично подтвердить не могу

Value encoding offers better performance for columns that are used in aggregations while hash encoding is preferred for group-by-columns (usually columns from dimension tables).

When Analysis Services starts processing a table, it will analyse some samples from each numeric column in order to determine which type of encoding will be applied. It is possible that, after the column is partially processed, based on new incoming information, it is necessary to restart the process to change the encoding previously defined. In order to avoid that, given your prior knowledge about the data, it is good practice to set your preference concerning the encoding method to be applied. For that, you just need to choose in the “EncodingHint” property of the column the method that should be running.