AirPort Time Capsule wifi is suddenly very slow: Boom network load
Changing network parameters like channel assignments might cause more harm than good, especially on the long turm. You may end up in having to analyse strange issues...

Use a professionnal tool to analyse your wireless environment first:
iStumbler.

If you want a professionnal help, please put a copy of a screen capture with this tool. Make this environmental measure at a point which is as near as possible from the point you want to communicate with your Time Capsule usually.
Take care to make this measure at the hour you witness your slow network throughput.
Sort the output along decreasing signal level (Level column).

Take care to hide private information from this screen capture.

If you have a real environmental problem (which a real and highly frequent situation), fix it.

Else, bring back your Time Capsule to your Apple reseller. Most probably an antenna cable was not correctly fixed inside, and a move made it unplug.