Shell Programming and Scripting

I have the following problem:
I have two files: S containing sentences (one in each row) and W containing files (one in each row). It might look like this:

S:
a b c apple d.
e f orange g.
h banana i j.

W:
orange
banana
apple

My task is to replace in S all words that appear in W with "X". How can I do that efficiently, if both W and S are large (100K lines)?

The output should look like this:
a b c X d.
e f X g.
h X i j.

Could you help me please?

awk:

Code:

awk ' FILENAME=="W" {arr[$0]++}
        FILENAME=="S" {
            for(i=1; i<=NF; i++) { 
                 if($i in arr) {
                     printf(" X ") 
                 } 
                 else {
                     printf("%s ", $i  )  
                 }    
            }
            printf("\n")
        } ' W S > newfile

Code:

awk 'NR==FNR{a[$0];next}{for(i=0;++i<=NF;){$i=($i in a)?"X":$i}}1' W S > newfile