let measure = async () => {
    // create some random data in a small buffer
    let smallbuffer = new Uint8Array(50000)
    crypto.getRandomValues(smallbuffer)

    // create a large buffer (10MB) with this random data
    let length = 10000000
    let buffer = new Uint8Array(length)
    buffer.forEach((_,i) => { buffer[i] = smallbuffer[i % 50000] })
    
    // call our function
    let start = performance.now()
    await base64_arraybuffer(buffer)
    let stop = performance.now()
    
    // return a measurement
    let seconds = (stop - start) / 1000
    let megabytes = length / 1000 / 1000
    return `MB per second: ${Math.floor(megabytes / seconds)}`
}

// example use:
await measure()
// returns: 'MB per second: 261'