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

[msilvestrixdatanet]

• 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)

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @kuanlu95 @mattgotteiner.