if you really want a deep dive (cuz your post suggests that), fire up wireshark on a speed test PC and capture the traffic while you test. look for out-of-order, missing and corrupted packets. you will also get awesome stats on the traffic - wireshark is your best friend. be warned, this is the entrance to the rabbit hole.
edit: because at some point you are going to want to slide wireshark between the cable modem and your router - for general troubleshooting (and funsies!) then things get interesting as you figure out how to do that properly.
holy shit! the FBI is communist?! cool, cool.