Just to be clear, the hardware I have is WRT1900ACSv2 not WRT1200AC. Have you tried this:
Patch the firmware for 88w8864 (mwlwifi specific high latencies) by disabling tx_amsdu.
Add in luci > startup > local startup (nano /etc/rc.local) the following commands:
echo "0" >> /sys/kernel/debug/ieee80211/phy0/mwlwifi/tx_amsdu
echo "0" >> /sys/kernel/debug/ieee80211/phy1/mwlwifi/tx_amsdu
You will also get much better performance if you do this:
Install irqbalance. Change 'enabled' from '0' to '1' in '/etc/config/irqbalance'.