Preloading strategy slow down when using with Angular universal app (SSR)

Question

I am using lazy loading for modules. But when the user navigates to the lazy loaded module route, it takes a couple of seconds to load the module.
So I use a custom preloading strategy with 20 second time delay. When I serve the application using "ng serve". Everything works fine. My website loads fast, lazy loaded module loads after 20 seconds, and there is no delay when the user navigates to the lazy loaded module route.
BUT When I serve the application using angular universal command "npm run dev:ssr", everything goes bad. Nothing loads until 20 seconds. My app takes more than 20 seconds to load every time. The amount of delay I put to delay preload, is applied to the initial load.

app-routing.module.ts

import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import {CustomPreloadingStrategyService} from './core/services/custom-preloading-strategy.service';
const routes: Routes = [
  {
    path: 'quick-links',
    data: {preload: true, delay: 20000},
    loadChildren: () => import('./modules/quick-links/quick-links.module').then(m => m.QuickLinksModule) 
  }
];

@NgModule({
  imports: [RouterModule.forRoot(routes, {
    initialNavigation: 'enabled',
    scrollPositionRestoration: 'enabled',
    preloadingStrategy: CustomPreloadingStrategyService
})],
  exports: [RouterModule]
})
export class AppRoutingModule { }

custom-preloading-strategy.service.ts

import { Injectable } from '@angular/core';
import { Observable, of, timer  } from 'rxjs';
import { map } from 'rxjs/operators'
import { PreloadingStrategy, Route } from '@angular/router';
@Injectable({
  providedIn: 'root'
})
export class CustomPreloadingStrategyService implements PreloadingStrategy {
  preload(route: Route, loadMe: () => Observable<any>): Observable<any> {
    if (route.data && route.data['preload']) {
      var delay:number=route.data['delay']
      console.log('preload called on '+route.path+' delay is '+delay);
      return timer(delay).pipe(
      map( _ => {
        console.log("Loading now "+ route.path);
        return loadMe() ;
      }));
    } else {
      console.log('no preload for the path '+ route.path);
      return of(null);
    }
  }
}

For reference, here's the PreloadAllModules strategy

Pieterjan
– Pieterjan

2022-09-08 18:08:48 +00:00
Commented Sep 8, 2022 at 18:08 — Pieterjan
– Pieterjan, Commented Sep 8, 2022 at 18:08

Akshay · Accepted Answer · 2022-09-08 14:34:01Z

Yesss. I found the reason for the slow loading.

The code written in "custom-preloading-strategy.service.ts" is executed on the server as well every time I reload the browser tab. This code should only be executed on the browser.
So, I put a new condition, and now code in "custom-preloading-strategy.service.ts only executes on the server.

custom-preloading-strategy.service.ts

import { Injectable, Inject, PLATFORM_ID } from '@angular/core';
import { isPlatformBrowser } from '@angular/common';
import { Observable, of, timer  } from 'rxjs';
import { map } from 'rxjs/operators'
import { PreloadingStrategy, Route } from '@angular/router';
@Injectable({
  providedIn: 'root'
})
export class CustomPreloadingStrategyService implements PreloadingStrategy {
  constructor(
    @Inject(PLATFORM_ID) private platformId: any
  ){}
  preload(route: Route, loadMe: () => Observable<any>): Observable<any> {
    // Add a new check here
    if (isPlatformBrowser(this.platformId) && route.data && route.data['preload']) {
      var delay:number=route.data['delay']
      console.log('preload called on '+route.path+' delay is '+delay);
      return timer(delay).pipe(
        map( _ => {
        console.log("Loading now "+ route.path);
        return loadMe() ;
      }));
    } else {
      console.log('no preload for the path '+ route.path);
      return of(null);
    }
  }
}

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