How to solve macOS wifi speed fluctuation, low speed problem?

There may be a too many reason but for me, unknowingly, reason is mDNSResponder daemon.

On macOS Sierra version 10.12.4 (I faced same problem El Capitan, too), when I download something in high speeds, sometimes speed drops to 0kbps and after few seconds it starts to download in 100mbps again. This fluctuation occurs occasionally but when it occurs it stays until I restart the computer.

The reason behind this problem is mDNSResponder daemon. For resolve issue, you can;

- Open Activity Monitor
- Jump to Network tab (for finding mDNSResponder process easily)
- click on mDNSResponder
- click X button which left of to the info button.(Force process to quit)
- exit from the process using "Force Quit" button

Or, if you are a terminal guy,

ps aux | grep mDNSResponder
sudo kill -9 <pid>