What operating system are you using? I predominantly use Linux on my laptop that has a "Broadcom Corporation BCM4352 802.11ac" adapter in it. At 2.4GHz on Linux it's driver performance is woeful (but this is known), even with the slight speed improvement that resulted from the latest driver updates in ROS. I have heard other Linux Wifi drivers are lacklustre too.
Copying files in Linux Mint to my SAMBA file server gets about 3.5MB/s on a 20MHz channel. In Windows on the same laptop it's over 7MB/s and in the past in Windows I have had over 14MB/s on a 40MHz channel.
I'd suggest to really test your Routerboard use a clear 20MHz channel from a few metres away and use iperf2 or iperf3 to test the speed between 2 computers (one on the LAN and one on Wifi) and leave the Routerboard CPU to just do bridging/routing/wifi. And use Windows on the Wifi machine.
Also, while you are testing remove or turn off all other clients connecting to the same AP to remove airtime competition.
Here is a verbose dump of mine. I use only 802.11N, no RTS/CTS, and preamble is 'both'.
/interface wireless
set [ find default-name=wlan1 ] adaptive-noise-immunity=ap-and-client-mode allow-sharedkey=no ampdu-priorities=0 \
amsdu-limit=8192 amsdu-threshold=8192 antenna-gain=0 antenna-mode=ant-a area="" arp=enabled arp-timeout=auto band=\
2ghz-onlyn basic-rates-a/g=6Mbps basic-rates-b=1Mbps bridge-mode=enabled channel-width=20mhz compression=no country=\
australia default-ap-tx-limit=0 default-authentication=yes default-client-tx-limit=0 default-forwarding=yes \
disable-running-check=no disabled=no disconnect-timeout=3s distance=indoors frame-lifetime=0 frequency=2462 \
frequency-mode=manual-txpower frequency-offset=0 guard-interval=any hide-ssid=no ht-basic-mcs=\
mcs-0,mcs-1,mcs-2,mcs-3,mcs-4,mcs-5,mcs-6,mcs-7 ht-supported-mcs="mcs-0,mcs-1,mcs-2,mcs-3,mcs-4,mcs-5,mcs-6,mcs-7,mcs\
-8,mcs-9,mcs-10,mcs-11,mcs-12,mcs-13,mcs-14,mcs-15,mcs-16,mcs-17,mcs-18,mcs-19,mcs-20,mcs-21,mcs-22,mcs-23" \
hw-fragmentation-threshold=disabled hw-protection-mode=none hw-protection-threshold=0 hw-retries=7 \
interworking-profile=disabled keepalive-frames=enabled l2mtu=1600 mac-address=XX:XX:XX:XX:XX:XX max-station-count=\
2007 mode=ap-bridge mtu=1500 multicast-buffering=enabled multicast-helper=full name=wlan1 noise-floor-threshold=\
default nv2-cell-radius=30 nv2-downlink-ratio=50 nv2-mode=dynamic-downlink nv2-noise-floor-offset=default \
nv2-preshared-key="" nv2-qos=default nv2-queue-count=2 nv2-security=disabled nv2-sync-secret="" on-fail-retry-time=\
100ms preamble-mode=both radio-name=000C42FC9DD1 rate-selection=advanced rate-set=default rx-chains=0,1 scan-list=\
default secondary-channel="" security-profile=default ssid=XXXXXXXXX station-bridge-clone-mac=00:00:00:00:00:00 \
station-roaming=enabled supported-rates-a/g=6Mbps,9Mbps,12Mbps,18Mbps,24Mbps,36Mbps,48Mbps,54Mbps supported-rates-b=\
1Mbps,2Mbps,5.5Mbps,11Mbps tdma-period-size=2 tx-chains=0,1 tx-power-mode=default update-stats-interval=disabled \
vlan-id=1 vlan-mode=no-tag wds-cost-range=50-150 wds-default-bridge=none wds-default-cost=100 wds-ignore-ssid=no \
wds-mode=disabled wireless-protocol=802.11 wmm-support=enabled wps-mode=push-button
