After decoding OrcEncodedDataConsumer::decodeBatch, data is enqueued into a queue in LlapRecordReader. Queue limit for this queue is determined in LlapRecordReader. If it is minimal, it ends up waiting for 100ms until it gets capacity.
determineQueueLimit takes into consideration all columns though only few columns are needed for projection. Here is an example.
For this query, queue size predicted would be "138" as it takes into account all fields instead of just 2. This would causes unwanted delays in adding data to the queue.