Lets start with the basic
All Cable/ASDL modems/routes will only transfer data at 10/100Mbps have not found or seen one that will transfer data at 1000Mbps as yet.
If you have a 10/100/1000Mbps LAN card the speed between the router and the computer will automatically adjust down to 100Mbps but the card will keeping searching for a 1000Mbps connection, which slows down the download speed
It would be better if you change the transfer rate on the 10/100/1000Mbps LAN card from Auto to 100Mbps full duplex between the router and computer
Across a local network PC-to-PC if all the PC have a 10/100/1000Mbps LAN cards the transfer rate would be 1000Mbps,
If the correct is unable to handle the 1000Mbps transfer speed the Lan card will automatic downgrade to 100Mbps
Hope that answer you question