Check your internet contract for minimum data rate requirement. ISP is probably throttling you down to the minimum during peak periods.
Also, do a basic DSL Speed Test during the time in question.
Also to note, you have 3 devices sharing the bandwidth, so divide that by 3 as well if all are used concurrently.
(require min. 2Mb/sec (3Mb/sec recommended)) per device for SD. (More for HD)).
Also remember, your system, when online, is using your bandwidth in the background for other things like time adjust, system and App updates, facebook crap etc.
* ...and you thought math class was over!
/kens