Movies can download all at once, with one connection. Chess games may have 100 separate connections.
Lags!!!
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This is about disconnects but may help with lag too.
https://support.chess.com/customer/en/portal/articles/1444918-my-internet-connection-is-fine---why-am-i-getting-disconnects-
YouTube and streaming sites can use things like bufferring to smooth out intermittent lag and packet loss. Live chess is essentially a real-time conmection protocol and doesn't handle dropped packets quite as well.
This few days the server of chess.com was so lag. I can watch movie in youtube.com smoothly but i keep on disconnect in chess.com!! wish you guys repair it as soon as posible.. its a good side by the way.