ethernet splitter - how do these affect internet speed dumb question

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For some real-world data on a Windows computer, look at Task Manager, Performance, Ethernet and watch how much bandwidth your computer is using.  Run some network-intensive tasks (video, for example) and see how much bandwidth it is actually using.  Watch out for the fact that the scale on the histogram will dynamically change.  My bet is that the comments above are spot on: you'll likely not have any real issue with sharing 1G over a handful of devices.

There actually were some "Ethernet splitters", but they were quite different.  They let you split an 8-wire Ethernet cable into two 4-wire cables.  You'd install the devices at either end (by the switch and by the client) and it would let you connect two devices through one in-wall cable.  Since you were using 4 wires, it would run at 100M instead of 1G.  That was handy in the 100M days where you just needed an extra jack, but a poor solution with 1G networks.

thanks for all the help. i will make sure to keep the secondary machine in OFF position then unless using it.

"i will make sure to keep the secondary machine in OFF position "
That shouldn't be necessary in terms of available bandwidth unless it has some processes running that upload or download a lot of information.  A Windows update might tie up some download bandwidth, but I'd bet you'd not notice the impact.