arr_noshare.js

var http = require("k6/http");

var s =  [{"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"},
    {"user": "myuser", "password": "password12345"}];

exports.default = function () {
    http.post("https://httpbin.test.k6.io/anything", JSON.stringify(s[Math.floor(Math.random()*s.length)]))
}

arr_share.js

var http = require("k6/http");
var SharedArray = require("k6/data").SharedArray;
function generateArray() {
    return JSON.parse(open("./arr.json"));
}

var s = new SharedArray("something", generateArray);

exports.default = function () {
    http.post("https://httpbin.test.k6.io/anything", JSON.stringify(s[Math.floor(Math.random()*s.length)]))
}

The arr.json is 100k lines such as the one in noshare array. The reason why the one array is much smaller is that otherwise, it will use too much memory :D
Both were run with --compatibility-mode=base -u 1000 -d 30s --no-teardown --no-setup
The results averaged over 3 runs are :
no-share: 146%CPU (Really stable somehow) 1055-1064M (again really stable) 1937-1947/s iterations
share: 163-168% CPU and 1162-1222M memory usage 1899-1946/s iterations

I ran it once for 5minutes ... because I have to do other stuff:
no-share:143% CPU, 1346M 1982.689996/s iterations
share: 166% CPU, 1455M 1985.615058/s iterations