Performance Testing and Functional Testing are two important procedures to determine how effective a website or application will be for the user. However, they are two different concepts by explanation.
Performance testing allows for the validation of system behavior under different load conditions with purely automated results. Functional Testing verifies the software's accuracy and relies on a manual or automatic procedure for results.
Performance Testing often takes place in a production environment and many H/W facilities to increase load feed. On the other hand, Functional Testing doesn't need a testing environment, and the H/W needs are kept at a minimum.