Revert "[kbn/optimizer] disable parallelization in terser plugin (ela…

…stic#58396) (elastic#58421)"

This reverts commit ce10569.

packages/kbn-optimizer/src/optimizer/optimizer_config.ts

@@ -25,15 +25,6 @@ import { Bundle, WorkerConfig } from '../common';
 import { findKibanaPlatformPlugins, KibanaPlatformPlugin } from './kibana_platform_plugins';
 import { getBundles } from './get_bundles';
 
-function pickMaxWorkerCount(dist: boolean) {
-  // don't break if cpus() returns nothing, or an empty array
-  const cpuCount = Math.max(Os.cpus()?.length, 1);
-  // if we're buiding the dist then we can use more of the system's resources to get things done a little quicker
-  const maxWorkers = dist ? cpuCount - 1 : Math.ceil(cpuCount / 3);
-  // ensure we always have at least two workers
-  return Math.max(maxWorkers, 2);
-}
-
 interface Options {
   /** absolute path to root of the repo/build */
   repoRoot: string;
@@ -119,7 +110,7 @@ export class OptimizerConfig {
 
     const maxWorkerCount = process.env.KBN_OPTIMIZER_MAX_WORKERS
       ? parseInt(process.env.KBN_OPTIMIZER_MAX_WORKERS, 10)
-      : options.maxWorkerCount ?? pickMaxWorkerCount(dist);
+      : options.maxWorkerCount ?? Math.max(Math.ceil(Math.max(Os.cpus()?.length, 1) / 3), 2);
     if (typeof maxWorkerCount !== 'number' || !Number.isFinite(maxWorkerCount)) {
       throw new TypeError('worker count must be a number');
     }

packages/kbn-optimizer/src/worker/webpack.config.ts

@@ -248,7 +248,6 @@ export function getWebpackConfig(bundle: Bundle, worker: WorkerConfig) {
           cache: false,
           sourceMap: false,
           extractComments: false,
-          parallel: false,
           terserOptions: {
             compress: false,
             mangle: false,