If I have one computer inside with the main internet thing and another in the shed with the secondary internet thing, the second will have slower internet. True? Is that at all how it works?

depends on signal strength but internet speed should be equal.

If you are running the 802.11G platform which is 2.4GHz, there should be no measurable difference, UNLESS there is excessive distance involved, or other interference with the signal such as brick & steel commercial walls, etc.

I'm gonna take a stab and guess that you're asking if a wireless connection to a router inside the same building that you're in, will be faster than if it was connected to a router outside your house, somewhere else.

As already stated, distance and interference can cause a decrease in performance. New homes may have metallic insulation which may inhibit wireless signals. Great distances can cause weak signal and do the same. You'd only know for sure if you actually tried it. But yes, you're always better close and in direct Line of Sight of antennae trying to communicate. Just like any radio.