[Bug]: Why the multi-turn optimization always spend time although I turn off it?

Nobody has claimed this yet.

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### RAGFlow workspace code commit ID

#6854

### RAGFlow image version

nightly

### Other environment information

```Markdown

```

### Actual behavior



### Expected behavior

_No response_

### Steps to reproduce

```Markdown
chat and find it apparently much slower than retrieval test
```

### Additional information

_No response_

No files, tests, or entry points are identified. First reproduce the slowdown in chat versus the retrieval test with multi-turn optimization disabled, then trace where that setting is read and where the optimization work is invoked. Done means disabled optimization no longer adds the reported delay, with a regression check for both enabled and disabled behavior.

Written by the indexing model from the issue text.