scala - Fastest way to take elementwise sum of two Lists

I can do elementwise operation like sum using
Zipped function. Let I have two Lists
L1 and L2 as shown
below

val L1 =
            List(1,2,3,4)
val L2 =
            List(5,6,7,8)

I can
take element wise sum in following
way

(L1,L2).zipped.map(_+_)

and
result is

List(6, 8,
            10, 12) 

as
expected.

I am using
Zipped function in my actual code but it takes too much
time. In reality My List Size is more than 1000 and I have
more than 1000 Lists and my algorithm is iterative where
iterations could be up to one billion.

In code I have to do following
stuff

list =(
            (L1,L2).zipped.map(_+_).map (_ * math.random) ,
            L3).zipped.map(_+_)

size
of L1,L2 and
L3 is same. Moreover I have to execute my actual code on a
cluster.

What is the fastest way to take
elementwise sum of Lists in Scala?