Wireless means radio/electromagnetic signals. All sorts of devices put out signals that can interfere with that. Not just things that are supposed to be signaling, like a portable phone handset or a cell phone. If there's imperfectly insulated wiring in the wall, that could be creating interference. The amount might vary depending on how much current is flowing; if what's plugged in is a light or television that's only on part of the time (rather than, say, an electric clock), you'll have an intermittent problem.
You bring up a good point. It was less than a year ago when my Service Provider called me and said that they detected interference with my signal. They came out and checked my outside cable first and found a faulty cable, said that it looked like squirrel damage. They replaced the cable and that didn't fix the problem so they proceeded inside.
Inside they checked all the wiring, connections, outlets, what was plugged in (including microwave), etc.
They found two minor old connectors, which they replaced, and one major problem . . . the very old TV that I had in my bedroom.
SP tech said "Wow. Look at this." He had his meter thingy hooked up to the TV and I saw all these fuzzy dots flying all over his meter screen. He unhooked the TV and all those fuzzy dots settled down into a pretty straight line. It was really quite interesting.
After all was said and done (they were out for close to 2 hours, with no charge to me!:o) once the old TV was unplugged, I was amazed as to how much more speed I had on my internet.
Oh, and today? Internet access is, once again, zooming . . . in my usual spot.