Pasting plain text is very slow now

Repro:

1. Open src\vs\editor\contrib\suggest\test\browser\completionModel.test.ts
2. Select all
3. Copy
4. Create src\vs\workbench\services\suggest\test\browser\simpleCompletionModel.test.ts
5. Paste
6. Scroll up, 🐛 I frequently see a spinner when pasting, didn't this used to essentially be instantaneous?


v1.97 insiders took around 424ms from ctrl+v to the content showing


[insiders-trace.json](https://github.com/user-attachments/files/18641589/insiders-trace.json)

v1.89.1 stable took around 20ms


[stable-trace.json](https://github.com/user-attachments/files/18641585/stable-trace.json)

AFAICT plain text is the default for `editor.pasteAs.preferences` and we therefore should not seemingly be waiting on extensions or anything else to provide any information before adding the content to the buffer. This makes the editor feel so slow imo.