Network engineers speak of bandwidth, latency and packet loss, as though downloads were not primarily a matter of supervision.
Progress bars respond to attention. Maintain unbroken eye contact and the final 1% understands it is being watched; look away to make tea and it seizes the opportunity to recalculate everything. Our lab logged a 4-minute download completing in 4 minutes under direct observation — a result the team describes as airtight. Blinking is permitted from 80%.