Why is FastAPI's UploadFile Slower than Flask for Large Files?

FastAPI UploadFile Performance Compared to Flask

FastAPI's UploadFile can appear slower than Flask's upload functionality when working with large files. This is primarily due to the way FastAPI handles uploaded files.

Default Data Handling

FastAPI employs a SpooledTemporaryFile object with a maximum size of 1MB. When an uploaded file exceeds this limit, the data is written to a temporary disk file. This process can introduce performance overhead, especially for large files.

Flask's Advantage

Flask, on the other hand, loads the entire file into memory by default. While this can be impractical for very large files, it provides faster performance for smaller files.

Overcoming Performance Issues

To improve the performance of FastAPI file uploads, several options are available:

• Asynchronous File Handling: Define the endpoint with async def to avoid blocking. UploadFile methods should be preceded by await to ensure asynchronous execution.
• Incremental File Writing: Write the file incrementally using the file.writer() method of UploadFile. This approach prevents the entire file from being buffered in memory.
• Request Body Streaming: Instead of using UploadFile, access the request body as a stream using Request.stream(). This allows data to be processed in chunks, avoiding performance bottlenecks.

Conclusions

FastAPI's file upload mechanism may be slower compared to Flask for large files due to its default file handling strategy. However, by employing asynchronous writing or streaming techniques, FastAPI can achieve similar performance to Flask while maintaining its strengths in other areas.

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