Since this is an issue that has repeated with many users of the internet regardless of isp. Or even having identical hardware/routers...
It would seem that this issue is niether the router or the isp. But the ISP and user can do various things to temporarily restore fast speeds. These fixes Generally last until the pc reboots.
It also seems to coincide with the recent solar activity. Sometimes disconnecting the etherlines and powering down the router etc...then grounding the etherlines...Like taking static off a tv screen...simply touch it...Helps remedy the issues temporarily for a longer period as well.
There are many,many things that can influence transmission of data/power over copper. Or wifi.