A memory card is essential for all low end phones and necessary for backing up your data in case of damage to your phone. Despite how useful they are, they can seriously slow down your phone phone if they are almost filled up. It doesn’t matter the amount of your RAM or if your phone memory is not occupied. If your memory card is full, your phone will noticably slow down.
Why It Happens
Normally, everything visible on your phone comes from a directory stored on your phone. Even if you are streaming, the data has to be stored and decoded before being shown to you. All of this reading of files from the internal storage or memory card takes a toll on the speed of your device.
If there are more files on your device, it becomes harder to read them and your device will get slower.
This explains why your phone was very fast when you just got it. Always make sure your storage usage is below 70% for proper performance.