My router/modem sometimes appears to slow down. I unplug the router's power, have a cup of tea, and plug it back in again. I also turn off the destination device. And turn it back on. I think it is called IT 101. A less dramatic suggestion from Mr Google is simply to reset it. I find unplugging normally works.
However, lots of bits in the world of IT are just unknown. I was moving stuff around on the daughter's android phone the other day. Cable connected. There were gigabytes of photos. It seemed slow. I put in a newer and less used cable, and it tripled the speed. Wierd. A cable is a cable, innit?