There appears to be a bug in some of Linksys smart routers or Comcast. It is the ipv4 speed test slower than real speed. However, ipv6 is not throttled. Comcast speed test on ipv4 speed is always < 20m. Media Prioritization on or off has the same result. Some people suggest disable; however, WMM enabled is still required to enable > 54 Mbps, so this is not the solution we are looking. The cause is that IPv4 packets from the Comcast network are configured with DSCP 8, which Linksys smart routers will treat all package as low priority traffic and throttle their speed. ipv6 have correct DSCP so it is not affected.
If you are facing this issue and using stock firmware, you can use the small fix below:
1. reset Media Prioritization. In case you messed up any setting before.
2. enable Media Prioritization.
3. Add a New Application.
4. Add a name and input port range 1 to 65535.
5. Drag the new added name to the High Priority box and save.
6. Test your speed again and you should see the speed back to normal.