Why My Arris Router Router Slowing Lan Down from 240 to 50?

Please see this solution

May be Causes of , you have multiple devices connected to the internet through that router, and especially if you have active torrents, many cheap routers will not be capable of handling it. Try connecting only one computer and test the speed (without active torrents).

What kind of internet connection is that, DSL or another one? It could be an issue with the cabling through the house.

If UTP cables run in parallel with power cables and other signal cables, the crosstalk could be an issue. 

If I will not work then upgrading the firmware, did you try to reset the router? If not, kindly reset for 20 seconds and reconfigure the unit.

Once reconfigured, see if there's a difference. By the way, do you remember what firmware version you were using previously before upgrading?