Packet Length: Who Determines It?

I tried this in comp.os.ms-windows.networking.tcp-ip,
but no nibbles....
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Been playing around with something called LAN Speed Test - between my
desktop PC, through the router, and to a NAS box.

Back around June, I was getting what I thought were pretty good speeds:
300-500 Mbps write speeds and 350-650 Mbps read speeds. On all of
those tests, LAN Speed Test listed "Packet Length" as 1,000,000.

Recently I came back to LAN Speed Test, and have been getting much lower
speeds (as in 11-20 Mbps write and 60-90 Mbps read. For all those much
slower tests, LAN Speed Test shows Packet Length as 10,000.

I do not see any way to set Packet Length in LAN Speed Test's UI, so I
am guessing that somebody else determines it.

True? If so, who?.... and is there any hope for getting back to
1,000,000?
--
Pete Cresswell