If YouTube videos are lagging, then please try out the following steps for resolution -
a. Pause the video for 2 seconds and then try playing it.
b. The minimum internet speed required for playing YouTube in normal quality is 2Mbps and to play HD quality videos minimum speed required is 4Mbps.
c. Reboot your Wi-Fi router.
d. Try to check if the Wi-Fi router has been configured according to the settings mentioned in the link below- http://teewe.in/wifisettings.