Strong 1200 / MTC WR1201 wlan2.4 - low speed

Hi,
I am trying to use a Strong 1200 device as a wlan2.4 client. It seems something wrong with this device, can't get it working well. Any help is appreciated.

Here is what I have:
No mods and additional packages installed

~# cat /etc/banner
OpenWrt 19.07.4, r11208-ce6496d796
~# top
Mem: 37276K used, 86608K free, 324K shrd, 2524K buff, 7332K cached
CPU:   5% usr  15% sys   0% nic  43% idle   0% io   0% irq  34% sirq
Load average: 0.32 0.21 0.14 2/55 4540
~# iwinfo
wlan1     ESSID: "hidden2"
          Access Point: 8C:6D:50:91:xx:xx
          Mode: Client  Channel: 11 (2.462 GHz)
          Tx-Power: 20 dBm  Link Quality: 54/70
          Signal: -56 dBm  Noise: unknown
          Bit Rate: 135.0 MBit/s
          Encryption: WPA2 PSK (TKIP, CCMP)
          Type: nl80211  HW Mode(s): 802.11bgn
          Hardware: 14C3:7662 14C3:7662 [MediaTek MT76x2E]
          TX power offset: none
          Frequency offset: none
          Supports VAPs: yes  PHY name: phy1
~# iw dev
phy#1
        Interface wlan1
                ifindex 9
                wdev 0x100000003
                addr 8c:6d:50:91:8f:f0
                ssid hidden2
                type managed
                channel 11 (2462 MHz), width: 40 MHz, center1: 2452 MHz
                txpower 20.00 dBm
                multicast TXQ:
                        qsz-byt qsz-pkt flows   drops   marks   overlmt hashcol tx-bytes        tx-packets
                        0       0       0       0       0       0       0       0               0

~# cat /etc/config/wireless

config wifi-device 'radio0'
        option type 'mac80211'
        option channel '36'
        option hwmode '11a'
        option path 'pci0000:00/0000:00:00.0/0000:01:00.0'
        option htmode 'VHT80'
        option disabled '1'

config wifi-iface 'default_radio0'
        option device 'radio0'
        option network 'lan'
        option mode 'ap'
        option ssid 'OpenWrt'
        option encryption 'none'

config wifi-device 'radio1'
        option type 'mac80211'
        option channel '11'
        option hwmode '11g'
        option path 'pci0000:00/0000:00:01.0/0000:02:00.0'
        option htmode 'HT40'

config wifi-iface 'default_radio1'
        option device 'radio1'
        option network 'lan'
        option mode 'ap'
        option ssid 'OpenWrt'
        option encryption 'none'
        option disabled '1'

config wifi-iface 'wifinet2'
        option ssid 'hidden1'
        option key 'yyyyyyyy'
        option device 'radio1'
        option mode 'sta'
        option bssid '18:D6:C7:83:xx:xx'
        option network 'wwan'
        option encryption 'psk2'
        option disabled '1'

config wifi-iface 'wifinet4'
        option device 'radio1'
        option mode 'monitor'
        option ifname 'mon1'
        option disabled '1'

config wifi-iface 'wifinet5'
        option ssid 'hidden2'
        option key 'xxxxxxxx'
        option device 'radio1'
        option mode 'sta'
        option bssid '04:5E:A4:92:xx:xx'
        option network 'wwan'
        option encryption 'psk2'

I have tested tp-link 841 (EOM firmware, WISP mode) with much better results up to 20%, the same conditions.

Should I enable extra CPU for better performance?
or
make some extra settings I don't know about?

How can I improve it? Please help!

Hi! Do you have any positive developments on your speed issue?
I have also speed problems on wire (with stock firmware), down: ~240Mbit, up: ~250Mbit.
What are your experiences? With Open-WRT will be faster? Why is this so slow? The 1/4 of the promised speed...
I'd rather take it back to the store.
BR,
Zsolt