I've monitored the Luci GUI Realtime Graph. It stops updating during a speedtest, and the peak value increases from ~2 to ~3.
When run 'top -n 1' via ssh, 'sirq' increases from always < 20 % to > 90 % and top also stop printing each second.
Is anyone else experiencing this issue?
Is the RT-N56U hardware getting too old for SQM / should I get a new router?
On a R7800 (dual core @ 1.70 GHz), I did not observe a difference between 18.x and 19.x-rc2. For me, downstream is CPU-limited to about 150 Mbps using cake/higher with another setting (see the thread I linked at the end of this post.)
Are you certain the change is related to the version? Note that bufferbloat tests should be run a few times to get an average value. See this thread.
A current, MT7628N running at 580 MHz caps out around 75 Mbps with SQM, without wireless, without anything else running, and on the bench. I don't know your line speeds, so I don't know if, say, 25-50 Mbps is "plenty" for you (my guess as to the RT-N56U's capabilities), or a limitation.
rule of software developement:
if a regression costs less performance than one could gain by upgrading to the latest hardware, the regression is not fixed.
if it costs more, it is maybe fixed "if there is time available".
Actually, I may need to change my answer... When I have SQM enabled on 19.07.0-rc2, I am seeing much longer load times for webpages both on wireless devices and wired devices. When I disable SQM entirely, the slowness goes away. I will start a new thread so as not to hijack.
EDIT: and I will need a quantitative way to measure it.