Might be a router problem for some, not for me. I've got two Retina Macbooks at home, a 13" early 2013 and a 15" late 2013.
The 13" runs just fine on 10.11.3, the 15" model is unusable on that same version. I had to roll back to 10.11.1 for it to be functional again, none of the tips and tricks I have searched for around the web have made any difference; and I've tried a lot of stuff...
Scary thing is that the yet to come 10.11.4 doesn't seem to solve this issue - that is referring to feedback I've seen from beta testers that have encountered the same erratic wifi behavior since 10.11.2.
Maybe we should start listing the hardware hit by this plague. So far most people I have read about are owners of a late 2013 15" Retina Macbook like myself, but most people fail to disclose what computer they're using.
So for those who have not yet found a solution to their unbearable wifi connection, which Mac model are you using?
You sound awfully certain. Have you tried resetting it or are you just certain it cannot be the problem? The only reason I say this is because I had exactly the same thoughts for quite a while. I have 2 Macbook Pros (1 2013 retina, 1 2011 non Retina); 2 Macbook Airs (2012 2014); 3 27" iMacs; 3 21" iMacs. Only 1 of the 13" macbook airs exhibited the problem (2014) and I would boot back to the older 10.10 and it would work fine on that MBA. All had the same version of the OS (even the ones that I ran the betas on). So why not the OS? It probably does have something do with the combination of it and the Asus router upgrade--for me.
After reading the tip online to reset the router, I thought what the heck can I lose but a few minutes to set it back up? And it worked. Do all the updates and the do a factory reset. I just want to make sure that people don't spend of lot of frustrating weeks/months waiting on Apple to fix it, if you can do it yourself in minutes.