You could well be suffering from bufferbloat which is basically the technical term for lag.
Check this website to test your current score
You can then head to Luci (advanced settings) install luci-app-sqm, refresh the Luci gui and you should have "SQM QoS" listed under the "Network" tab (inside Luci only)
Enable the Plugin (check)
Enable this SQM instance
Interface name:
That should be whatever is (wan)
Do a speedtest over at nperf.com (do 3) take your average and deducted 15% off the overall speeds for download and upload. Put the values in the fields and apply (convert them to the correct values that are expected)
Then run the bufferbloat test again.
You are aiming at a A+ rating which I have managed to achieve with my Brume 2. The upload speed is what you want to focus on too.
If you see Download Active / Upload Active have increased + ms values when you are running the test then you need to adjust your speeds on Luci to be lower, for example if your download is testing and it jumps to +4 then that's acceptable, if your upload is the same then that's probably okay to get A+ rating.
The key is to make sure both are balanced when getting the results, dial both settings until the + value are as low as you can go without crippling your overall ISP speed as that is going to decrease due to the rules, try and get as much speed and the A+ rating as you can (usually just means dialing back the upload speed)
Keep all the other settings as they are and save and apply.
There is currently a SQM bug on some firmware and if you reboot the router the the settings for SQM might not be re-applied, you can confirm / test by re doing the bufferbloat test after a reboot to see if your result are worse again. If the SQM isn't applying then just login to Luci and enable / disable the checkmark on the SQM settings or go to: Luci > system > startup and find "sqm" then click restart, you will need to do that after each router reboot for now.
Good luck.
Edit -
Well I just realised the date on the original post, anyway it might help others or at least the people that replied prior.