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What is a bottleneck (why is my copy slow)?

In one sentence: The slowest part of a chain of steps, which sets the speed of the whole thing however fast the other parts are.

A little more

A bottleneck is the slowest step in a chain, and it decides the speed of the whole chain. Copying a file goes from the laptop's drive, through its USB port and the cable, into the stick's memory chips, and each file also needs an entry in the file list. Whichever of those is slowest sets the copy speed; making any other step faster changes nothing.

That's why the fix depends on the bottleneck. A fast stick in a USB 2 port is held back by the port (about 35-40 MB/s), a cheap stick is held back by its slow memory however fast the port is, and thousands of tiny files are held back by the per-file paperwork, which zipping them into one file removes. The same idea explains slow websites, slow Wi-Fi and laggy games: find the slowest link first.

See it in action

  1. BonusWhy is copying to a USB stick so slow?Send ants carrying crumbs from a laptop, over a bridge, into a USB stick and watch what really slows a copy down: a skinny USB 2 bridge, tired ants when the fast cache runs out, or paperwork for thousands of tiny files.10 min · interactive
  2. BonusWhy does the game lag when someone starts a download?Your internet line is a shop with one checkout and a download is a shopper with endless trolleys.9 min · interactive
  3. 3Can a website get tired?Why websites slow down or fall over when too many people visit at once.7 min · interactive
  4. BonusWhy does the internet slow down in the evening?Picture the internet as water: your house's pipe, one shared pipe under the street and a big main from far away.9 min · interactive

Do it for real

Free grown-up tools and guides on Tooltern, our sister site. They work in your browser and nothing is uploaded.

Related words

  • Bandwidth The most data a connection can carry each second, like the number of lanes on a road; internet plans are sold by it in Mbps.
  • Throughput How much data actually gets through right now, usually less than the bandwidth because of Wi-Fi, walls and other users.
  • SLC cache A small, fast part of a USB stick's or SSD's memory that new data is written to first, so copies start quickly and slow down once it fills.
  • Contention ratio How much bigger all the homes' plans added together are than the shared pipe they use, such as 1:20. It works because people rarely use their whole connection at once.

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