Partitioning, pros and cons

[Monument Bob 2]

I've read and heard conflicting reports on partitioning of hard drives. The pro opinions are that the whole process of sequestering various programs by establishing programs into different drives produces much greater efficiency and speed. The "yes, but" folks say that's true and if one has started establishing them from the first day of buying the computer, then fine to partition. But to attempt that once everything is on C:\ is just about impossible to carry out.
What's the definitive scoop?
Bob