Collapsed page tree is broken and very slow

**Bug number 1:**

> 1. In file 'node_modules/apostrophe/modules/@apostrophecms/page/ui/apos/components/AposPagesManager.vue' I set:
```js
export default {
name: 'AposPagesManager',
data() {
return {
treeOptions: {
startCollapsed: true, // <<
```
> 2. Start server: 'node app @apostrophecms/asset:build ; npm start'
> 3. Please watch Apostrophe app video:

https://user-images.githubusercontent.com/17063579/144985738-b9525582-4cdc-4fb7-84bb-37d0ff94b396.mp4

**Bug number 2:**

It takes tens of minutes to render an expanded 2000-page tree.
Rendering of a collapsed tree is not much faster.
The collapsed tree contents must be generated dynamically during the opening of the tree parts!

**Software:**

Node.js v16.9.1
Apostrophe v3.8.1
Chrome v96.0.4664.55 (x86_64)
macOS Big Sur v.11.6

Start with node_modules/apostrophe/modules/@apostrophecms/page/ui/apos/components/AposPagesManager.vue and reproduce the collapsed-tree behavior using `node app @apostrophecms/asset:build ; npm start`. Compare collapsed and expanded rendering against the linked video and a 2000-page tree; done means collapsed loading is responsive and tree contents are generated when branches are opened.

Written by the indexing model from the issue text.