Yes thanks for pointing that out, I wrote a slightly faster version below. I took your advice and wanted just a single invoke on the C program, so I modified it to take an argument. I'm satisfied!

    > cat arc4random-v2.c
    #include <stdio.h>
    #include <stdlib.h>
    int main(int argc, char** argv) {
      if ( ! (argc == 1 || argc == 2) ) {
        printf("usage: %s [n_output]\n", argv[0]);
      } else {
        int n_iter = (argc == 2) ? atoi(argv[1]) : 1;
        for (int i = 0; i < n_iter; i++) {
          printf("%u\n", arc4random());
        }
      }
      return 0;
    }

    > ./arc4random 3
    4282823399
    333796506
    762478899
    
    > cat pwgen-v2.sh
    #!/usr/bin/env bash
    dict=/usr/share/dict/words
    n=4 # of words to include in passphrase
    nl=$(wc -l $dict | sed 's_[ \t]*\([0-9]*\).*_\1_')
    fmt_flag() { echo $[$1 % $nl] | sed -n -e 's_^_-e _' -e 's_$_p_p' ; }
    flags=$(./arc4random $n | while read -r i; do fmt_flag "$i"; done)
    words=$(sed -n $flags < $dict)
    echo $words
    
    > time ./pwgen.sh
    liparite evade retiringly spacious
    
    real	0m0.268s
    user	0m0.251s
    sys	        0m0.015s
    
    > time ./pwgen-v2.sh
    latherin prideling thumbscrew unraveled
    
    real	0m0.082s
    user	0m0.072s
    sys	        0m0.013s

EDIT: note that the single call to sed results in alphabetical/dictionary-dependent results... (!) oops