In office are 20+ wifi divices. ISP gives 80Mbps internet traffic. Most download speed is 10Mbps from router.
Router: Mikrotik RB951Ui-2HnD
Flash: v6.0rc14
Router settings:

[admin@MikroTik] > export compact
# jan/02/1970 13:20:28 by RouterOS 6.0rc14
# software id = ILBI-F2A5
#
/interface bridge
add admin-mac=D4:CA:6D:E0:FA:DD auto-mac=no name=bridge-local protocol-mode=\
rstp
add l2mtu=1598 name=test-bridge protocol-mode=rstp
/interface ethernet
set 0 l2mtu=2028 name=ether1-gateway
set 1 name=ether2-master-local
set 2 name=ether3-slave-local
set 3 name=ether4-slave-local
set 4 name=ether5-slave-local
/interface wireless
set 0 band=2ghz-b/g/n basic-rates-a/g=6Mbps,9Mbps,12Mbps,18Mbps,24Mbps \
basic-rates-b=1Mbps,2Mbps country=ukraine disabled=no distance=indoors \
ht-rxchains=0,1 ht-txchains=0,1 hw-protection-mode=rts-cts l2mtu=2290 \
mode=ap-bridge periodic-calibration=enabled \
periodic-calibration-interval=10 radio-name=D4:CA:6D:E0:FA:E1 ssid=\
wifi-name supported-rates-a/g=6Mbps,9Mbps,12Mbps,18Mbps,24Mbps \
supported-rates-b=1Mbps,2Mbps tx-power=21 tx-power-mode=all-rates-fixed \
wds-default-bridge=test-bridge wds-mode=dynamic wireless-protocol=802.11
/ip neighbor discovery
set ether1-gateway discover=no
/interface wireless nstreme
set wlan1 enable-polling=no
/interface wireless security-profiles
set [ find default=yes ] authentication-types=wpa-psk,wpa2-psk eap-methods=\
passthrough group-ciphers=tkip,aes-ccm mode=dynamic-keys unicast-ciphers=\
tkip,aes-ccm wpa-pre-shared-key=1qaz@WSX wpa2-pre-shared-key=1qaz@WSX
/ip hotspot user profile
set [ find default=yes ] idle-timeout=none keepalive-timeout=2m
/ip pool
add name=default-dhcp ranges=192.168.88.10-192.168.88.254
/ip dhcp-server
add address-pool=default-dhcp disabled=no interface=test-bridge name=default
/interface bridge port
add bridge=test-bridge interface=ether2-master-local
add bridge=test-bridge interface=wlan1
add bridge=test-bridge interface=ether3-slave-local
add bridge=test-bridge interface=ether4-slave-local
add bridge=test-bridge interface=ether5-slave-local
/ip address
add address=10.10.10.10/24 disabled=yes interface=ether1-gateway network=\
10.10.10.0
add address=192.168.88.1/24 interface=test-bridge network=192.168.88.0
/ip dhcp-client
add comment="default configuration" dhcp-options=hostname,clientid disabled=\
no interface=ether1-gateway
/ip dhcp-server network
add address=192.168.88.0/24 comment="default configuration" gateway=\
192.168.88.1
/ip dns
set allow-remote-requests=yes
/ip dns static
add address=192.168.88.1 name=router
/ip firewall filter
add chain=input comment="default configuration" protocol=icmp
add chain=input comment="default configuration" connection-state=established
add chain=input comment="default configuration" connection-state=related
add action=drop chain=input comment="default configuration" in-interface=\
ether1-gateway
add chain=forward comment="default configuration" connection-state=\
established
add chain=forward comment="default configuration" connection-state=related
add action=drop chain=forward comment="default configuration" \
connection-state=invalid
/ip firewall nat
add action=masquerade chain=srcnat comment="default configuration" \
out-interface=ether1-gateway src-address=192.168.88.0/24 to-addresses=\
0.0.0.0
/ip route
add distance=1 dst-address=10.10.10.0/24 gateway=ether1-gateway
add distance=1 dst-address=176.120.33.5/32 gateway=wlan1
/queue interface
set wlan1 queue=default
/system leds
set 0 interface=wlan1
/tool mac-server
add disabled=no interface=ether2-master-local
add disabled=no interface=ether3-slave-local
add disabled=no interface=ether4-slave-local
add disabled=no interface=ether5-slave-local
add disabled=no interface=wlan1
add disabled=no interface=bridge-local
/tool mac-server mac-winbox
set [ find default=yes ] disabled=yes
add interface=ether2-master-local
add interface=ether3-slave-local
add interface=ether4-slave-local
add interface=ether5-slave-local
add interface=wlan1
add interface=bridge-local

You are using very old RouterOS version save your configuration and update to latest 6.32.3 and wireless-cm2 packaged.

Then try to find frequency that is least used in your office try to avoid interference that could be cause of slow wifi. Try this http://wiki.mikrotik.com/wiki/Manual:Spectral_scan .

Set 20/40mhz channels.

And check if your client devices have 2 chains and 40 MHz channels enabled to achieve maximum speed.

You are using very old RouterOS version save your configuration and update to latest 6.32.3 and wireless-cm2 packaged.

Then try to find frequency that is least used in your office try to avoid interference that could be cause of slow wifi. Try this http://wiki.mikrotik.com/wiki/Manual:Spectral_scan .

Set 20/40mhz channels.

And check if your client devices have 2 chains and 40 MHz channels enabled to achieve maximum speed.

Thanks for response.

Also, i have problem with firmware update.
Error: could not resolve address upgrade.mikrotik.com

You are Welcome,

add DNS servers

/ip dns set servers=8.8.8.8

but I suggest to do clean install with netinstall http://wiki.mikrotik.com/wiki/Manual:Netinstall

because you have sold old version.