Memory type for variables

I found a configuration in which the only difference is the local keyword before the rows variable. This uses a little more resources, but the difference in performance (time) is more significant. It is significantly slower, 980us where the non-local runs in 700 us. I think this means the addition of local prevents the compiler from inferring a shift register. If that is the case I assume the first piece of code infers registers and thus is private. Am I correct?