> The multipart streaming workload is inherently expensive.

Streaming workload is not inherently expensive. The main work is to bring the bytes of the files to the network card as quick as possible, and nearly no computation needs to be performed.

> The cost of generating boundaries and constructing headers scales with request count and payload size.

The only computation necessary to generate boundaries is to ensure that the chosen boundary does not occur in the content, and it seems that the code does not actually check this, but generates a random UUID4. Boundaries and headers are per-file and could be cached, so they don't scale with the number of requests or payload size.