Would having 2 computers connected wireless to a router slow down the other computers connection and speed?

Ok, So I have a computer upstairs which is connected wireless to the router down stairs. The computer down stairs is connected through an Ethernet cable from the router. My computer, the wireless one. Is very fast on the internet. Now my parents have moved the computer that was down stairs to upstairs and want to... show more Ok, So I have a computer upstairs which is connected wireless to the router down stairs. The computer down stairs is connected through an Ethernet cable from the router. My computer, the wireless one. Is very fast on the internet. Now my parents have moved the computer that was down stairs to upstairs and want to get a wireless card for that computer. What i am wondering is, will having another computer connected wireless to the router slow down my computers connection because of the more packets getting sent and traffic etc...
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