My theory relates more to a problem we used to see in NA with “grouped streams”. Specific geographical areas share bandwidth. If enough users are chewing up bandwidth the higher rate users are limited to ensure an equitable share to the other users on the same switch. Since most users in NA are now connected to ISPs that provide 2,3,4 or more GB fibre service “equitable bandwidth sharing” has, for the most part, been eliminated. I am seeing a LOT of UK users that are connected at 100MB and less. if there is ANY sharing this could drop you well below acceptable levels during high load periods.
Check out this bit of research. Very enlightening.
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