The geometry of the mesh gets distorted when rigged and posed I tried to set up a rig into my character as you can see in the image. However, I have two problems :
A. When I move the rig, the geometry gets distorted crazy.
B. Whenever I move any joint, the pose mode lags so slow. All the joints do not bend as smooth as they are supposed to be in Youtube tutorials. Here are my approaches to the problem, which did not work :

Since the body mesh had 'Multiresolution' modifier on, I thought it was the excess number of geometry that is causing all the lagging and making the computer difficult to process. So I deleted the multires modifier and tried it with same rig. However, both the lagging or the crazy distortion of the model did not disappear.

I did not tried the weight painting method in rigging which most blender tutorial demonstrates, because just like the pose mode, the weight paint mode lags crazy. Maybe that is the problem, but I will give the description of how it looked when I entered weight painting mode: The character was in full dark blue with no red or green colours. Would there be any other approaches to solve this problem? By the way this is the tutorial that I followed : https://www.youtube.com/watch?v=TL45rxB30_o


Also, here is the file of my mesh : https://drive.google.com/file/d/1URSnA5C7Wp66RLt6UQEupGX9waX3DQpk/view?usp=sharing
So to fix your problem:

Disable the Multiresolution modifier, because it makes your computer lag (before parenting you need to bake the multires normals so that you can use a low-poly object instead).
Apply the object scale with CtrlA &gt; Scale (explanation about object scale here).
In Edit mode select all and press ShiftN to recalculate the normals.

To avoid parenting bugs it's always better to apply scale and rotation and to make sure that the normals are correctly flipped.
The file is here.