How to improve the rig in blender to prevent weird mesh deformations?

Not directly an unity question, but yeah, but about the character I am going to use in unity. So, I generated a character in makehuman and imported it blender. I have neither used Mhx rig or rigify as they will decrease performance due to lots of bones. I just used simple HumanIK rig. But, the rig doesn't contain any IK bones, so I created them myself. But, now I am having a problem that the deformations of mesh are not that natural. On rotating upper shoulder character rotates in weird way and on rotating hip the character hip mesh gets deformed. It is hard to explain the exact things happening. Just the problem I said. So, is there Any blender user who has good experience with rigging? I just need few suggestions to improve my character rig :/ ? You can get the .blend file from here: https://www.datafilehost.com/d/01d07ddd