How to Reduce the lag?

That depends on what you are doing in your game.

If you create a simple scene with a plane to walk on and a character controller and nothing else, is it still a problem?

If not, then you need to check your scripts for something that takes a lot of time to run.

Its likely the rendering of game graphics, or a script. It could also be you have a lower end computer. Mine is powerful, but unity limits the Frames Per Second to about 60. First, do you have multiple Update functions in you're scripts? This can cause lag/fps drops. If you also have too many While()s, it can also create lag.

Go into the game window and click "Stats" and a small window should pop up. Now, run the game. If you get about 10 fps, then its likely the games graphics taking up power, or like I said, the Updates. If it freezes, it could be either, but likely the scripts. This has happened to me several times. Try changing function Update() to LateUpdate(), and adding
yield WaitForSeconds(5); Around the script (if its JavaScript), and also disabling some scripts.

This may not work, but you can also try this.[Keep in mind, that you shouldn't rely on the below code, always program games to not take up performance] :

Create a js file and insert this:

 #pragma strict
 
 function Start () {
 Application.targetFrameRate = 300;//Target the frame rate to about 300fps
 QualitySettings.vSyncCount = 0;//This is vsync. This just tells unity to sync framerate to monitors refresh rate. 
 //Turn off vSync for better fps, by turning QualitySettings.vSyncCount to 0
 }

Another option if you have a less powerful computer, is to turn graphics down. Edit -> Project settings -> Quality, then click fastest. I think the freezing is a script thing though.

I am so sorry for the late answer, but the reason that happens is because realtime global illumination is turned on, realtime gi doesn't really work good with slow computers...