`unit-test` builder with `vitest` is slow for testing and for code coverage

### Command

test

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

I migrated a very small existing project using the jasmine-to-vitest schematic and code coverage is fairly slow when using vitest (roughly 80 tests).

With the `karma` builder:

```
ng test --no-watch 9,99s user 1,71s system 214% cpu 5,459 total
ng test --no-watch --code-coverage 17,82s user 2,42s system 232% cpu 8,719 total
```

When migrates to the unit-test builder (and a few tests disabled):

```
ng test --no-watch 18,49s user 3,33s system 259% cpu 8,396 total
ng test --no-watch --coverage 38,46s user 4,66s system 210% cpu 20,535 total
```

The tests themselves takes 18s (10s with Karma), and the code coverage takes 20s instead of 8s. This is very noticeable when using vitest.

### Minimal Reproduction

This is probably reproducible in a new CLI project, by generating a bunch of components and comparing test times.

### Exception or Error

```text

```

### Your Environment

```text
Angular CLI : 21.0.0-rc.1
Angular : 21.0.0-rc.1
Node.js : 22.18.0
Package Manager : npm 10.9.0
Operating System : darwin arm64

┌───────────────────────────┬───────────────────┬───────────────────┐
│ Package │ Installed Version │ Requested Version │
├───────────────────────────┼───────────────────┼───────────────────┤
│ @angular/build │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/cli │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/common │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/compiler │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/compiler-cli │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/core │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/forms │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/localize │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/platform-browser │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/router │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ rxjs │ 7.8.2 │ 7.8.2 │
│ typescript │ 5.9.3 │ 5.9.3 │
│ vitest │ 4.0.8 │ 4.0.8 │
```

### Anything else relevant?

Of course this is maybe an upstream problem, but I thought it was worth mentioning it.

Start by reproducing the reported timings with the `ng test --no-watch` and coverage commands in a small Angular CLI project using the `unit-test` builder and Vitest, then compare them with Karma. Read the unit-test builder and the jasmine-to-vitest schematic entry points implicated by the migration. Done means identifying the source of the slowdown and demonstrating improved test and coverage timings.

Written by the indexing model from the issue text.