Hi, sorry for my english 
RB951G-2HnD RouterOS 6.28
I have no option vlan1 channel-width 40Mhz Turbo.
I connected my phone to provider router wifi, my speed was 50Mbs, on my home router RB951G-2HnD download speed is only 20Mbs on phone and 30Mbs on notebook.
update
Another phone work well 50Mbs speed
but why my phone only 20Mbs? on another router download speed 50Mbs
most smartphones do not supports 40mhz channels or dual spatial stream, because that the throughput its limited to 20-25 mbps
use the wireless fp package to show wich channel width its registered client using
selected item is my phone
i test this phone on anoter router. download speed over 60Mbs
ok yuri
on your screen shot we can see your phone with signal of -79 to get optimum speed you need to be at least at -63 like the device listed there.
activate the ccq column to see the ccq of clients
When I do the measurement speed, I keep the phone close to the router, and the last time as well.
my phone cpu snapdragon 400 msm8926 (info from cpu-z)
wpa or wpa2?
tkip or aes?
do the test without crypt.
without crypt
previous tests i use wpa2 and aes
are you sure your smartphone supports 40mhz channel?? many smartphone do not
in the screen we can see another device reporting 40mhz channel, it confirms accesspoint its supporting it
I come with the phone to my mother. She router from the provider. There is also the speed of downloading 40-50Mbs. I can not look at this router on what frequencies the phone operates. If this speed can only be 40MHz, then my phone exactly the support 40MHz.
Any ideas? RouterOS bug?
remember, on 2.4ghz the use of 40mhz channel has certain requirements of coexistence.
http://www.smallnetbuilder.com/wireless/wireless-features/31743-bye-bye-40-mhz-mode-in-24-ghz-part-1
thanks for the link, I analyzed the Q1-Q4 article
Q1: What are the conditions for an AP to fall back to 20 MHz?
Q2: Is the fallback done on a frame by frame basis or by static mode switch?
Q4: Do non Wi-Fi sources, i.e. Bluetooth, baby monitors, microwaves trigger fallback?
Q5: How do I know if my router will fall back?
I have on this access point running on 40MHz devices, as far as I understand this is not my case
Q3: Does the neighboring network need to be active?
I ran a speedtest on the phone to check
How can I check?
Q1: What are the conditions for an AP to fall back to 20 MHz?
2) A client device with its 40 MHz (“fat channel”) intolerant bit set is detected
An example of another cause for fallback is that if an 802.11n station sends a “20/40 BSS Coexistence” management frame containing one “20/40 BSS Intolerant Channel Report” element to AP. If the report includes a channel in its channel list that is within the +/- 40 MHz of its set channel, the AP will fall back to 20 MHz mode.
Update to 6.29.1
WiFi speed became worse.
If another phone is connected to WiFi, then the rate of my phone was less than 15-17Mbs, before about 20Mbs. If i turn off WiFi on the other phone, then my phone speed 20Mbs as before.
Another phone download speed over 45Mbs.
I usually check the speed at a distance of 1 meter from the router. If I check speed by touching the phone to the router, the speed it falls to 5-7Mbs. if we measure the speed on the other phone touching the to a router, it is not the speed falls.
its all about CCQ and data-rates at client level and you know what performance to expect
check possible interference