在数据库中建立关系会使它们变慢

I have just started internship at a small software house and I am working on an ERP. My team lead has forbidden me to create any relationships in the database. As this is my internship I was shocked because till now I have read that relationships are necessary for ensuring data integrity. My team lead told me that we can enforce the data integrity at the front end. After some research I found out that foreign keys do make the db slower but Indexing foreign keys can increase performance.I have just started internship at a small softw