app_preloading_strategy.ts

export class AppPreloadingStrategy implements PreloadingStrategy {
    preload(route: Route, load: Function): Observable<any> {
        const loadRoute = (delay) => delay
            ? timer(150).pipe(flatMap(_ => load()))
            : load();
        return route.data && route.data.preload 
            ? loadRoute(route.data.delay)
            : of(null);
      }
}

Where do I gettimer, flatMap, and of from? I've not worked with these before and I see no import statement.

rxjs? lodash?

May I recommend when making example code, please use universally understood code without extra implicit code

import {PreloadingStrategy, Route} from '@angular/router';
import {Observable, of, timer} from 'rxjs';
import {flatMap} from 'rxjs/operators';