Your son is hogging the bandwidth and your router is working on a "First come, First served" basis.
The answer, in part, is to use a Wireless Router that has the QoS, Quality of Service, feature, something that the Time Capsule does not.
A QoS router can prioritize traffic and assign more bandwidth, up to the available amount, to those programs that are susceptible to lag. VOIP and gaming for instance.
Newer routers are even able to differentiate between a large file download and a video stream and balance the load out in favor of the video stream.
Nothing above really helps you as the primary problem is your lack of bandwidth. With only 5Mbps to play with, on a good day, it only takes one video download and almost all of your bandwidth has gone.
Have you looked into the Satellite alternative. They have come a long way in the last few years.
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Ok... I have an Airport Time Capsule fed by a fairly slow 5Mbps/1Mbps connection. I have an iMac, Directv W.H.DVR and a Brother printer connected by ethernet. I connect via WiFi, a MacBook Pro, Mac Mini, a Windows 8 PC, a Samsung Smart TV 2 iPad mini's, my sons home theater system (rarely used) and a Windows 8 laptop. At most, 3 devices are used at one time. One of my issues; When my son is watching a movie or videos in his room, my internet is severely slowed on my MacBook Pro. I would like to give my MBP and iMac priority over any other device on the network. I guess there's not a way to do this with the time capsule? Would a hardware firewall do this, or do I need a load balancer of some kind? What do I need to get to solve my problem? I live in the country where there is no internet option, other than satellite, or my ISP. My ISP is a wireless internet provider. We have a "radio" mounted under the eave of the roof. When I call my provider about issues, he says he runs a speed test to the radio and "makes adjustments" today he got me from 3Mbps to 5Mbps. 3 was very bad, but 5 is a little more tolerable. I'd really like to get the most out of what I have by prioritizing. Sorry this is long, but I just need to be pointed in the right direction. Any ideas?

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