No one can advise with any accuracy without knowing exactly what you're trying to run on the system. This model is a budget- to lower mid range consumer-grade notebook -- it is fully capable of being used for general purpose consumer applications (web browsing, office work, viewing videos) but it's never going to work well with video encoding, gaming, or anything involving heavy-duty CPU or I/O work.
If it's just a memory issue, you can upgrade this system to 64 G max.
If it's more than that -- such as that it's overheating and throttling, you need to choose a system with a heavier duty cycle such as a Precision workstation or a true gaming notebook.
Memory upgrade:
https://www.crucial.com/compatible-upgrade-for/dell/inspiron-15-(5518)
