So I have been struggling with this for over a year. In my house we have 4 Macs of one for or another and an Apple Airport Extreme base station with 802.11/ac. My laptop is the only one with 802.11/ac the rest are 802.11/n. I am the only one who has any trouble.
My setup is: Computers -> Airport Extreme in Bridge Mode (but that doesn't actually matter) -> Virgin Media cable modem (in the UK) in router mode.
I can ping my airport extreme's IP address with zero packet loss. When I ping the cable mode just on the other side of it (i.e., connected by ethernet cable as one does), I get close to 50% packet loss. When I cannot ping my cable modem I also cannot reach the internet. I definitely cannot make new connections but sometimes I think that existing connections are still working.
While suffering from this problem, and pinging various endpoints continuously, I can successfully ssh into my wife's laptop on the same in house network, and run ping on her laptop, and it pings without dropping any packets both the internet and the cable modem.
If I run "netstat -an | grep SYN_" I often see tons of TCP connections in SYN_SENT mode, which means they are trying to establish a connection but getting no answer. For a year at least I thought THAT was the issue and it never occurred to me to see if I could actually ping my cable modem. So this long term SYN_SENT symptom is just a side-effect of the actual underlying networking problem.
If I shut off wifi and turn it back on, all the stuck connections are fixed, and pings work again for a while. If I shut off wifi, delete 4 or so airport configuration/preference files, reboot, then turn wifi back on and reconnect to my airport, everything works flawlessly for several days.
Today I did a SMC reset and a PRAM (I think) reset with a senior apple support person on the phone, and removed one of my applications just in case it was causing trouble (iStat Menus - doubtful as all my other computers are in fact running the same software), and since that reboot 2 hours ago my laptop has been fine, and I am getting blazing speeds (150 mbits/second from virgin, the rated speed).
I have opened a bug with apple and am in contact with them. They have suggested (within the last week) that this issue might be one that they will have a fix for in the next major release. If they have a fix now, how dare they not put it in El Capitan, after all, this is wifi networking, it's supposed to work!
Meanwhile, I left off the best piece of information: I have two routers in my house. If I connect to the other router, which only does 802.11/n and is in general way slower than the Apple one, I drop no packets. None. 100% success rate. It's a buffalo brand, nothing special. I usually use it to VPN in to the United States.
So this bug is literally between me and the airport extreme base station, except actually it's between me and the cable mode IFF I connect via the Apple router (fine if it's another router instead), it's the same if I am in front of the router in the same room or on the other side of the house, it's only me and nobody else in the house (but again, only I have ac networking) and if I reset a few files on my computer and reboot, the problem disappears for days at a time.
It seems like people here might be having the exact same problem: when you experience packet loss, things get slow :-) If you are dropping packets when you click on a link in the web page, it might seem slow until all of a sudden it's fast, and that's the same thing: dropping packets until suddenly not dropping them for a few seconds or minutes.
I hope Apple fixes this. So many things about the Apple software and the OS in particular are just dead-on perfect, but things like this can ruin an otherwise great experience.
JP
PS: Forgot to mention that I have a Retina MacBook Pro 15" with 802.11/ac from Mid 2015. Also forgot to mention that within the last two weeks I did a fresh OS re-install w/out restoring from Time Machine backup just in case that was the issue. Problem re-appeared within 3 to 5 days.
