n++ being a unary operation, it just needs one variable. Whereas, n = n + 1 is a binary operation that adds overhead to take more time (also binary operation: n += 1). However, in modern platforms, it depends on few things such as processor architecture, C compiler, usage in your code, and other factors such as hardware problems.
While in the modern compiler even if you write n = n + 1 it will get converted into n++ when it goes into the optimized binary, and it will be equivalently efficient.
