Cards can affect the phone for the following reasons:
1. Cheap/imitation cards that have poor performance.
2. Damaged/faulty/old cards.
3. Corrupt cards. The most common cause of corrupt cards is people disconnecting the phone from a computer (whilst in mass storage mode) without first selecting the "safely remove hardware" option.
4. Apps that run at startup or apps that constantly run installed to the memory card. This can cause slowdown in some cases.