Suddenly slow speeds

I noted my internet connection running rather slowly in the last few days. My internet service delivers 800mb/s and my modem can handle 600mb/s. I used to get ~500 on my wired pc, but it dropped to less than 50 on speedtest.net

To see if it was just my pc being dumb I copied the speedtest-netperf.sh script over and ran it.

Download: 173.05 Mbps
  Latency: [in msec, 60 pings, 0.00% packet loss]
      Min:  21.578
    10pct:  22.576
   Median:  26.027
      Avg:  27.424
    90pct:  32.588
      Max:  44.222
 CPU Load: [in % busy (avg +/- std dev), 55 samples]
     cpu0:  97.0 +/-  2.7
 Overhead: [in % used of total CPU available]
  netperf:  65.4

So that's not good. I power cycled everything and there was no difference. Unfortunately, this is the extent of my troubleshooting skills.

What should I be looking at to sort this out?

Frequently the ISP is the culprit if the speeds changed and you haven't made any changes on your own network.

Unplug your router and plug a computer directly into the internet connection, run a speed test. Bypassing the router will be the best way to determine if the ISP is at fault. Keep in mind, though, that your computer will be directly exposed to the internet, so be sure you have the firewall enabled and set properly.

Monthly data usage exceeded ?

Plugging a laptop directly into the modem gets me ~530Mb/s, which is ok enough for me.

It seems like netperf.bufferbloat.net isn't cooperating so I can't test directly on the router itself, but my pc speeds through the router are back down to sub-50Mb/s.

What does the link speed between your router and your pc report? If it isn’t reporting 1Gbps/full duplex, check your cables.

connecting your laptop directly to the modem will get all the ISP's DNS and gateway settings.
are you using a custom dns provider with your router? is your router routing IPV6 or not?

i ask because i recently had a similar situation with a router using opendns with just IPV4, and a mysterious slowdown on the ISP (spectrum) that also mysteriulsy reverted itself when the router got a new IP address after 3 days. Numerous modem and router reboots had not fix the issue.

Similar situation here. I'm running a Linksys EA8500 (ARM) and noticed my speed went from over 800Mbps between WAN and LAN to around 320Mbps when I 'upgraded' from 19.07 to 21.02. I downgraded and got the 800+ Mbps back. I bought another EA8500 to investigate. With the factory 21.02 build and the only config change being to set the password, I got 335 Mbps. I reflashed it to the factory 19.09 build changing only the password and got 800+ Mbps again. When I run top during an internet speed test, ksoftirqd is chewing much more cpu on 21.02 than on 19.09.

Again, with vanilla out-of-the-box config on an EA8500, 21.02 reduced my internet bandwidth by more than 50%.

Thread on this issue started here -

Something like this has just happened to me, I am trying to fix it right now. I am experiencing slow and unstable speeds and a much higher latency than usual. Using Archer C6 V3 with OpenWrt SNAPSHOT r20686-f0ea3df439. It started today's morning without any reason. Multiple reboots didn't fix it, but if I connect my PC directly then it's as fast as it was before it started.