Xfinity internet speed drops from normal speeds to low speeds minutes after modem restart

I have a 100Mbps internet plan, and I am using a  NetGear CM400 modem. Internet speed always starts >100Mbps right after modem restart. Then after 10-15 minutes, it goes to about 10Mbps.  This issue happens both when using Wifi Router and when connecting to modem directly using an ethernet cable.

I have seen that many users are having similar issues. 

My signal levels are as below when the modem is directly connected to data line with ~10ft cable without any splitter. Does anyone know if these signal levels are reasonable?  Given that at first the speeds are high and expected, is it possible that this is due to some kind of throttling on the ISP side? Is there a way to figure this out?