Why wont allocating more RAM to Java/Minecraft make it run better/faster?

I upgraded my RAM not to long ago to 8GB. I found a tutorial on youtube of how to allocate more RAM for Java so minecraft can "run better." Well, I think I did it right but it just doesn't seem to run better than before. Could this maybe be my GPU? CPU? Video Card? My cpu isn't the greatest. Could these affect the performance?

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