Windows 7: Do inactive devices slow network speed

I have a custom built PC running windows 7 home premium, an i5 2500k, and 16gb of 1600mhz RAM, and I'm occasionally experiencing lag while watching Youtube videos in firefox. I scan regularly for viruses with malwarebytes and superantispyware, so i know thats not the problem, I run ccleaner about once a month thus keeping browser bloat to a minimum. But we do have 4 devices that are connected to the wifi, 3 phones and an ipod touch. Can these devices slow the network even if they are not being used?

Of course they can, as can garage door openers, microwaves, etc.

Whats the network setup? Is the slow down from machine to machine when transferring files, or is it on t he internet?

What is you ISP supposed to be giving you?

I have a 12mps connection speed that clocks regularly at 11.5mps, even when I'm having the problem. The desktop that I gave the specs for and am having the problem with, has a wired internet connection, and only has problems (that I have observed) with youtube videos.

and the four other devices are?

Two Droid bionics a pantech breakout an an ipod touch