Scala Creating combination by Key efficiently - scala

Question

Update
Original file is a text file that is in a form of
( record_id, element )
( 1 1 2 3 )
( 2 2 5 6 7 )
I used the following function to read input file:
sc.textFile(input)
and process it into the records array format below.
=========================================================================
Here is a sample an array in scala im working in
Records Array looks like below:
Array(Record_Id , Array(Element) )
( 1 , Array(1,2,3 ) )
( 2 , Array(2,5,6,7) )
....
I wrote a scala map function to extract prefix(dynamically in here i take first half of the elements) of each array
val prefix = records.map(x => ((x._1, x._2) ,(x._2.take((x._2.size*0.5).ceil ))) )
Which will give me
Array(Record_Id , Array(Element) , Prefix)
( 1 , Array(1,2,3 ) , Array(1,2))
( 2 , Array(2,5,6,7) , Array(2,5))
....
Now I try to generate a RDD that uses each prefix as key, Which would look like something like this:
(Prefix , Array(Record_Id , Array(Element) )
( 1 , (1 , Array(1,2,3 ) )
( 2 , (1 , Array(1,2,3 ) )
( 2 , (2 , Array(2,5,6,7) )
( 5 , (2 , Array(2,5,6,7) )
....
I tried the following:
val pairedWithKey = prefix.map{case (k,v) => v.map(i => k ->i)}
It works perfectly on small volume data.
However, once i use this in a larger datasets it will take forever to load.
I'm still new to scala, it would be great if anyone can direct me on how to improve the performance of this operation.