If your phone is not optimized, it can make your phone heavy and slow. This can be the result of several factors, but the most common one is the excessive use of memory. 

A lot of apps are using memory for storing data that should be saved. For example, when you open a photo or video in the app, it will consume memory. You can reduce the memory usage by deleting apps that are rarely used. This can provide extra storage space, which will make your phone lighter and faster. 

You can also optimize your phone by killing background apps.