Very slow upload speed on 5GHz ac connection

With your Mac mini connected to the 802.11ac AirPort Extreme by wireless, option-click on the Wi-Fi menu in the OS X menu bar.

Please provide the following values:

• Security
• Channel
• RSSI
• Noise
• Tx Rate
• PHY Mode
• MCS Index

Thanks for the numbers.

A couple of things stand out:

• The signal quality, at the mini, is somewhat low. -79 - (-93) = 14. You would want to have that value to be 25+.
• The MCS Index of 4 (the maximum value would be 9). With an index of 4 the theoretical throughput for a single stream for a 80 MHz channel is: 175-195 Mbps. Although 802.11ac allows for up to 8 simultaneous streams, Apple currently supports 3-4 for their devices ... so 195 x 3 = 585 Mbps would be the maximum with an MCS Index of 4. (Note: Both devices would need to support the same number of streams.) Possibly why you are seeing the 500-550 values at times as I'm not sure which model of the Mac mini you have.)

The bottom line to all of this is that you mini is not getting the best possible bandwidth for your wireless network. This, most likely, is being caused by Wi-Fi interference or placement.

I have a reinforced concrete wall on way from AE to my mini. I will try measure speed with iPad Air 2 when I get back home, from different places.

Yes, that would be an issue, as far as, affecting signal quality. It's best to use wired connections (where possible) in this situation.

Right now I changed position of Mini a bit, the SNR is now 39 and MCS is now 6. Speedof.me gives me same results, Speedtest.net gives a bit better results on upload, but much lower on download.

MCS of 6 is much better, and so is your "new" SNR value. I would suspect that you would also be getting better results with both upload & download speeds. One other thing is to look into using different DNS servers, especially if you are using the ones provided by your ISP.

So, as to my understanding, it is quite useless to "play" with channes and stuff, right? And why download speed is "full scale"?

Anything is possible, but changing the connection to Ethernet would provide the best results. Not sure why, on wireless, you are getting poor performance on uploads, other then DNS as I mentioned. One application that can help determine if changing DNS servers will help is namebench.