I wouldn't think so, no. If userspace isn't using it, then there's no latency overhead; the kernel access its own internal data structures the same regardless. If userspace is using it, the kernel still uses its own data structures just the same, but now there's a way for programs to read those data structures. Using the procfs driver doesn't change anything inside the kernel (well, anything important; obviously it exercises the procfs driver code (which also will have its own variables) and there's some data structures that describe mounted filesystems), it just provides an interface to what's already there.