Why is FastAPI's UploadFile Slow Compared to Flask?

FastAPI UploadFile is Slow Compared to Flask

Issue

Uploading a 100 MB file using FastAPI and Python requests takes approximately 128 seconds, significantly slower than the 0.5 seconds it takes when using Flask.

Response

The difference in upload time is primarily due to the way data is handled in FastAPI compared to Flask. When handling file uploads, FastAPI utilizes a SpooledTemporaryFile with a default maximum size of 1 MB for incoming request data. When the file size exceeds 1 MB, the data is written to a temporary file on disk, resulting in a slower upload process.

Potential Solutions

1. Synchronous File Writing

Define the endpoint using the regular def keyword instead of the async def keyword and write the file synchronously.

2. Asynchronous Writing (Using aiofiles)

Define the endpoint using the async def keyword and use aiofiles for asynchronous file writing. This approach requires awaiting the file operations to complete.

3. Using request.stream()

Access the request body as a stream using request.stream(). This method allows chunks of data to be processed without storing the entire body in memory or on disk, improving upload performance.

Key Recommendations

For optimal performance when uploading larger files, consider using the request.stream() method. This approach bypasses the 1 MB maximum size limitation and allows for direct processing of data chunks without additional write/read operations.

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