Windows theme is not faster, and will cause a lot of problem

I've always been under the impression that the Windows Classic theme was faster and thus always changed to it shortly after getting a new computer. Needless to say, I've been pulling my hair out recently because my programs have been so slow. I found something online and someone had mentioned trying the basic Aero theme. It has been a night and day difference, I feel like I'm using a computer and not a potato. As to why this happens, maybe someone here can answer?