If you experience slow download speed with Linksys smart router in Comcast network, you are encountering a bug in some Linksys smart routers. When it happen, ipv4 speed test will be slower than real speed. However, ipv6 is not throttled. Comcast speed test on ipv4 speed is always < 20m.
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: