Your router might be running old firmware. Visit the router manufacturer’s website and download / install the latest firmware. In most cases, this resolves jitter and lag related issues.

It is also possible that your connection cannot handle the bandwidth. Try connecting your computer to the network with an ethernet cable and check if the performance is better. If you're on a corporate network, there could be some kind of bottlenecking going on by limiting bandwidth per device.

Make sure to restart your router, Mac, and device you want to mirror at least once.

Try changing the preferred mirroring resolution (under the mirroring tab) to 720p and mirror again. 

For even better performance, but lower quality, select the "slow network" option under the mirroring tab. Then mirror again.

Use iStumbler (Mac) to see which Wi-Fi channels are in use around you, then configure your router to use a channel that is free. You should preferably also configure your router to only use Wi-Fi-n or Wi-Fi-ac (if supported).

Make sure the device you want to mirror to AirServer is within sufficient range of your Wi-Fi.