It’s probably the best you will get with the old wireless driver. If you want even faster 5GHz speed (500+ Mbps) you may want to test the new wifi driver, but this requires building a new, slightly different wifi configuration from scratch.
Maybe the signal strength was too high(!) in that case, due to the selected channel. Try to moving away some meters from the AP.
I feel very Olympic today, is there a guide how to do this?
Hmmm …
maybe start here and then see how far you get 
Going back to channel width, how do I decide which one of these to use:
My assumption is that if auto frequency was suboptimal, the XXXX control channel auto is also suboptimal and I should chose one of the “C” options. Which one?
The C stands for Control channel and every C or e (or X) is 20MHz bandwidth.
In case of Ceee and channel 5500:
5490-5570 used frequencies, where the control channel is frequency 5500.
In case of eCee and channel 5520:
5490-5570 used frequencies, where the control channel is frequency 5520.
A tool like inSSIDer will show exactely what is used in these cases.
In this topic you can find some more info:
http://forum.mikrotik.com/t/understanding-channel-width-options-and-limitations-due-to-country-registrations-plus-interface-vs-capsman-settings/151239/1
iperf3 up and running: https://gist.github.com/zupo/852c0aceeaeb9ba3fe68c7d58b413895
Wired:
[ ID] Interval Transfer Bitrate Retr
[ 8] 0.00-10.00 sec 1.08 GBytes 930 Mbits/sec 70 sender
[ 8] 0.00-10.00 sec 1.08 GBytes 930 Mbits/sec receiver
Wireless (Tx Rate 866):
[ ID] Interval Transfer Bitrate Retr
[ 8] 0.00-10.00 sec 748 MBytes 628 Mbits/sec 0 sender
[ 8] 0.00-10.02 sec 747 MBytes 626 Mbits/sec receiver
Wireless (Tx Rate 866) with an iPhone running speedtest next to the MacBook, to add some noise
[ ID] Interval Transfer Bitrate Retr
[ 8] 0.00-10.00 sec 550 MBytes 461 Mbits/sec 300 sender
[ 8] 0.00-10.01 sec 549 MBytes 460 Mbits/sec receiver
Pretty awesome 
Anything above 300mbps, I’m happy.
Nice !
630Mbps on cAP AC is something which could be only dreamed of some years ago.
If you have a capable client device and a real AX AP, you can go up to 700-750Mbps.
But for that difference I would not replace that cAP AC right now, especially not with new wave2 drivers 
Why is that? Did the firmware improve over the recent years?
FWIW, I’m running RouterOS 6.49.10 LTS on my cAP AC.
OP, you installed the wifi-qcom-ac driver package already (needs ROS 7.13+)? Either way - your iperf3 results look pretty good.
Wait a moment, I was under the assumption you already applied wave2 drivers to that cAP AC ?
So … you’re pushing 630Mbps on cAP AC using ROS6 ?
That’s hard to believe.
Theoretically you can only get data rate (=866 Mbps) divided by 2 as rough estimate for real speed = 433Mbps or about and that’s on wave1 radios.
Using wave2 drivers (ROS7.13 at least and qcom-ac driver), you can get to 700-750Mbps.
I have two more cAP ACs a few meters/walls away to cover the rest of the first floor. I need to choose channels for them.
Looking at https://en.wikipedia.org/wiki/List_of_WLAN_channels, this is my thinking:
You can also go down to channel 36 (=5180) since your all alone
No, haven’t switched to wave2, wanted to see what the local maximum is on the old driver, before I go and rip everything apart.
Maybe I am doing some measurements wrong? See screenshots.
Granted, these are the best results. I don’t consistently get 600Mbps, most of the time it’s around 400Mbps.
Ah nice, fair point, overlooked that one, thanks!
36,52,68,84
I would try DFS channels on the cAPs that need maybe cover a slightly larger area. Channel 100+ have higher transmit power (but need to adhere DFS).
I get “bad band or frequency” if I try to set 68. I guess country policy limitation?
Should be 36 , 52, 100, 132 (5180 / 5260 / 5500 / 5660 respectively)
68 is not allowed for 40/80 width, only 20.
Definitely not in the list of allowed 80MHz channels according to that earlier linked Wikipedia article.
Thanks holvoetn for correcting my bad!
Why not 116? On https://en.wikipedia.org/wiki/List_of_WLAN_channels it seems that it should be good too?
132 does not seem to work though, I get “bad band or frequency” when I try to set frequency=5660
Fun fact, I also have a wsAP ac lite, and with the same settings as cAP ACs above, I can’t get above 100mbps. Also fiddled around with different channel-widths, distances, etc. Won’t go over 100 mbps. Is cAP AC just that much better hardware?