Slow Wireless Network via Time Capsule
Any WiFi has a bandwidth. 

That is the maximum amount of data it can handle.

For example the 5 GHz WiFi can handle up to 8x more data then the 2.5 GHz.

Setting (limiting) a bandwidth usage for a specific application is not easy and Mac does not offer a build in solution, but there might be some external applications for that.

Here are some suggestions how to do that.

However, one can also use the AirPort Utility (in your utility folder) to set the Date/Times for Time Capsule. Set it to Times when you are not using the computer to prevent overcrowding your WiFi network.