How can I improve my cable modem/router’s wireless throughput?

1. If you have a Zoom Model 5352 cable modem/router, please see this answer.
2. Especially if you’re in a neighborhood with other wireless routers, use your router’s built-in Configuration Manager to set the wireless bandwidth to 20 MHz, NOT 40 MHz. (As an alternative, you should be able to use 40 MHz if OBSS Coexistence is Enabled.)
3. You can also use the router’s Configuration Manager to check that the Control Channel is Auto or another channel with an acceptable interference level.
4. You can try adjusting the antenna position(s) of the router or whatever is wirelessly connecting to it. At least one of the router’s antennas should normally be vertical.
5. You can try putting the router on a metal surface, which increases the effective length of the antennas.

In testing your wireless performance, you can check the download speed by using Speakeasy or one of the other free broadband modem speed testing sites. Speakeasy is at www.speakeasy.net/speedtest/ . You can compare this performance to the performance of something plugged directly into one of your router's LAN ports.

This How-To Geek article has good information on finding the best Wi-Fi channel for your router:
http://www.howtogeek.com/197268/how-to-find-the-best-wi-fi-channel-for-your-router-on-any-operating-system/