Significant slowdown in `SearchClient.upload_documents(...)` for large payloads

- **Package Name**: azure-search-documents- **Package Version**: 12.0.0- **Operating System**: Windows 11/Ubuntu 24.04- **Python Version**: 3.12**Describe the bug**After upgrading `azure-search-documents` from `11.6.0` to `12.0.0`, we observe a significant slowdown in `SearchClient.upload_documents(...)` for large payloads (hundreds of docs, vector field dimension 3072).A similar issue happens when invoking `SearchClient.merge_documents(...)`.The regression appears client-side, before HTTP/network becomes dominant.**To Reproduce** 1. Create two clean virtual environments: - one with `azure-search-documents==11.6.0` - one with `azure-search-documents==12.0.0` 2. Use the same Azure AI Search service and the same existing index for both runs. - The index must contain a vector field (e.g. `content_vector`) with dimension `3072`. - Keep all index settings identical across runs. 3. Prepare a synthetic payload with large vector-heavy documents: - hundreds or even thousands of documents per run (e.g. 1000) - each document includes: - key/id - text field (thousands of chars) - vector field with `3072` floats - a few metadata fields 4. Run the benchmark **serially** (not in parallel), alternating versions: - run 1 with `11.6.0` - run 2 with `12.0.0` 5. For each run, measure: - wall-clock time of `SearchClient.upload_documents(documents=payload)`**Expected behavior**12.0.0 should not introduce a major regression compared to 11.6.0 for typical bulk upload workloads with large vector fields.**Additional context**In `12.0.0`, `IndexDocumentsBatch._extend_batch` builds each action as:action_dict = {"@search.action": action_type}action_dict.update(doc)action = IndexAction(action_dict)This goes through model conversion/serialization paths (```Model.__init__, _create_value, _serialize```) for each document and recursively for nested structures (including large vectors).Likely files/methods:- ```azure/search/documents/models/_patch.py (IndexDocumentsBatch._extend_batch)```- ```azure/search/documents/_utils/model_base.py (Model.__init__, _serialize)```

---
Tracked in AB#5586937 (temporary link test; work item will be deleted after verification).