Uverse makes all applications on my macbook run slow--sometimes

To a tech whizz, I bet this issue will sound like a feather-brained hallucination but I have tried very many reasonable, practical solutions (I will list them all,) and it endures.

In a nutshell: the wifi set-up I have at home is making my computer slow. To be clear, it’s not that my wifi itself is running slow. All my applications, moving through the finder, clicking on the apple menu, all slow down. I get the cheerful pinwheel, eternally bouncing icons in the dock, or nothing at all.

What I’ve found is that if I switch off wireless and wait up to five minutes the problem goes away AND I find that the issue does not occur at all when I’m on other networks (like, say, at the apple store, which I’ve been visiting a lot, or a coffee shop, because I guess I'm a stereotypical apple user.)

Also, this always occurs after I start up. But after I’ve gone through this once, it usually doesn’t recur. I’ll fire up wifi after five minutes, and be good to surf and do whatever as long as I want. (Occasionally, it will recur after a few hours, and I’ll have to run through the same solution.)

I’m using Yosemite 10.10.5 on a MacBook Pro, 13-inch, early 2011. I have 8 GB of memory, and a 2.3 GHz Intel Core i5 processor. I’m on ATT Uverse, with a DSL Modem, specifically a Pace Model 5031 NV-030.

I promised I would list the things I’ve tried:

Repair disk from recovery mode

Hardware test.

A hardware check thing they do at the genius bar.

Re-install the operating system from recovery mode.

Delete the entire hard drive, and migrate all my files individually from Time Machine.

Create a seperate User Login and see if the problem persists there.

Check activity monitor for patterns (a lot of kernel_task)

Watch to see if there is any correlation with overheating. Does the mac run slow specifically when it is hot (implying a broken fan.) (Sometimes yes, sometimes no. There’s no correlation.)

Upgrade from 4 GB of memory to 8 GB of memory.

Install and run a virus checker (It found nothing, and I took it off again.)

Pressing the reset button on the router.

Delete all my preferred networks (I read online somewhere that it might be searching through all of those.)

So, please, I would love to try more stuff. It is tremendously frustrating that I can’t solve this. I greatly appreciate any thoughts you might have.