Minor Description
In the DataFrame ALS prediction function, we use blas.sdot which may be slower due to the conversion to Arrays. We can try operating on Seqs or another data structure to see if avoiding the conversion makes the operation faster. Ref: https://github.com/apache/spark/pull/17090/files/707bc6b153a7f899fbf3fe2a5675cacba1f95711#diff-be65dd1d6adc53138156641b610fcada
