Multiple files nearly simultaneously uploaded after a delay

I try to send multiple files to a Google Cloud endpoint using a drag and drop module react-dropzone.

For each file, I call a redux-saga that calls the fetch to the Google endpoint.

As visible in the screenshot below, all requests from fetch are done almost simultaneously but after a delay (40 seconds in my example). My question is: why not every request every 10 seconds?

I guess it's because of the reader.readAsArrayBuffer(file), but I'm not sure and can't figure out why.

function DicomDropzone() {
  const onDrop = useCallback((acceptedFiles) => {
    for (let file of acceptedFiles) {
      const reader = new FileReader()
      reader.onabort = () => console.log('file reading was aborted')
      reader.onerror = () => console.log('file reading has failed')
      reader.onload = () => {

enter image description here

Thanks for your help!

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