Another thinkg many people overlook....
Over time, CABLES GO BAD....
When I was working data entry, we had to help our tech guy replace about 10 cables per month in a 50-PC work area. Luckily, we didn't have to replace the ones running through the drop ceiling that often, but in the desk/computer station environment (and it's probably true in a lot of home setups), movement of the computer/router/modem, cables lying where they can be stepped on or accidentally hooked by feet under the desk, environmental factors and the like make the short cables tend to wear out much quicker than you'd think.
My system had been lagging a little more each month, so I replaced a 2-year-old, 1 meter long, cable between my router (note my router and modem are both newer than that cable - my computer too, for that matter) and my computer, and got a 10-20 ms improvement in my connection speed to my shard (returning it to where it was when the old cable was still relatively new, and I got the current computer and router).