Generating browser application bundles takes a long time (351616ms)

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**

Generating browser application bundles takes a long time (**351616ms**)

**Expected behavior:**

It should take much less time

**Steps to reproduce:**

**Related code:**

```
ngx-admin@8.0.0 start
> ng serve

⠴ Generating browser application bundles (phase: building)...WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($spacer, 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

╷
298 │ $headings-margin-bottom: $spacer / 2 !default;
│ ^^^^^^^^^^^
╵
node_modules\bootstrap\scss\_variables.scss 298:31 @import
src\app\@theme\styles\styles.scss 12:9 root stylesheet

DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($input-padding-y, 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

╷
493 │ $input-height-inner-quarter: calc(#{$input-line-height * .25em} + #{$input-padding-y / 2}) !default;
│ ^^^^^^^^^^^^^^^^^^^^
╵
node_modules\bootstrap\scss\_variables.scss 493:80 @import
src\app\@theme\styles\styles.scss 12:9 root stylesheet

DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($custom-control-indicator-size, 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

╷
560 │ $custom-switch-indicator-border-radius: $custom-control-indicator-size / 2 !default;
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
╵
node_modules\bootstrap\scss\_variables.scss 560:49 @import
src\app\@theme\styles\styles.scss 12:9 root stylesheet

DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($spacer, 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

╷
706 │ $nav-divider-margin-y: $spacer / 2 !default;
│ ^^^^^^^^^^^
╵
node_modules\bootstrap\scss\_variables.scss 706:37 @import
src\app\@theme\styles\styles.scss 12:9 root stylesheet

DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($spacer, 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

╷
711 │ $navbar-padding-y: $spacer / 2 !default;
│ ^^^^^^^^^^^
╵
node_modules\bootstrap\scss\_variables.scss 711:37 @import
src\app\@theme\styles\styles.scss 12:9 root stylesheet

WARNING: 20 repetitive deprecation warnings omitted.

✔ Browser application bundle generation complete.

Initial Chunk Files | Names | Size
vendor.js | vendor | 7.40 MB
styles.css, styles.js | styles | 2.87 MB
main.js | main | 2.02 MB
scripts.js | scripts | 1.30 MB
polyfills.js | polyfills | 636.78 kB
runtime.js | runtime | 12.86 kB

| Initial Total | 14.22 MB

Lazy Chunk Files | Names | Size
src_app_pages_pages_module_ts.js | - | 21.42 MB
src_app_pages_extra-components_extra-components_module_ts.js | - | 4.32 MB
src_app_pages_modal-overlays_modal-overlays_module_ts.js | - | 2.55 MB
src_app_pages_tables_tables_module_ts.js | - | 1.64 MB
src_app_pages_maps_maps_module_ts.js | - | 1.61 MB
src_app_pages_forms_forms_module_ts.js | - | 1.57 MB
src_app_pages_charts_charts_module_ts.js | - | 1.20 MB
src_app_pages_ui-features_ui-features_module_ts.js | - | 1.18 MB
src_app_pages_layout_layout_module_ts.js | - | 1.13 MB
src_app_pages_editors_editors_module_ts.js | - | 584.12 kB

Build at: 2022-05-14T18:47:16.545Z - Hash: 33cb3affab11ece6cd10 - Time: 351616ms

** Angular Live Development Server is listening on localhost:4200, open your browser on http://localhost:4200/ **

√ Compiled successfully.
```

### Other information:

**npm, node, OS, Browser**
```
Node: v16.15.0 , npm: 8.5.5 Angular CLI: 13.3.5
OS: Windows 10 Enterprise 64-bit (10.0, Build 19042) (19041.vb_release.191206-1406)
Browser: Google Chrome Version 101.0.4951.54 (Official Build) (64-bit)
```

**Angular, Nebular**
```
"version": "8.0.0",
same configuration https://github.com/akveo/ngx-admin/tree/feat/material-theme
```