For a table GUID uses for primary key and when we use it as a primary key so it creates a clustered index on that table. When we use GUID as clustered index then it decreases performance for inserting new data.
because there is 16 bytes binary data which is logically grouped by "-" sign.
4bytes 2bytes 2bytes 2bytes 6bytes
So its size 4+2+2+2+6 = 16bytes
You won't get much help if you don't start accepting some answers people give you.