FETCH STREAMS ARE GREAT, BUT NOT FOR MEASURING UPLOAD/DOWNLOAD PROGRESS

Summary from the TLDR Web Dev newsletter

While fetch streams are useful for processing data, they shouldn't be used to measure upload/download progress because they give inaccurate results. This is because they measure when data is buffered rather than actually transmitted over the network. For now, XMLHttpRequest is still the best option for progress tracking, though a new fetch observer API is being developed to properly address this gap.