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Then why is the 100 kB test the slowest, with a max speed of 115 Mb/s? And no upload test gets above 250 Mb/s.

The speed is very clearly throttled by the ISP though, they sell a faster package that I don't need. I'm not complaining, I've just never seen this burst behavior anywhere else.

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This is because of the way tcp congestion control works. In order to achieve higher speeds, tcp has a "window" that grows, essentially the amount of data in flight, that has not yet been acknowledged by your client. That window doesn't open very wide in the 100kb test because it happens so quickly.

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