Processing mode property of a partition/measure group determines how partitions will be available to users.
Processing mode has two possible options.
For those of you who likes technical lingo, Process Full will internally run Process Data and Process Index before the partition can be queried. If processing mode is set to Lazy Aggregations, partition will be released for user queries after Process Data is completed. Process Index will be executed in the background.
Faster availability of the partition comes with a price. Since aggregations don’t exist while users begin to query the partition they may experience slow performance.