Or, to put it another way, will your application be bigger, smaller, slower, faster, sexier, etc.... Generally, at least at this stage in the life of Harbour, any given body of code that can be compiled with Harbour and with CA-Clipper, on the same machine and with the same environment (it wouldn't be much of a comparison otherwise would it?) will run slower and will probably be "bigger" when compiled with Harbour.
The same is probably true when comparing Harbour compiled code with other Clipper compatible compilers.
The reasons for this are many and varied and, as is pointed out in many places in this document, Harbour is still early in its life and continuing development and Clipper compatibility are of greatest importance. The issue of "performance" will be addressed when it is most appropriate to do so, and when people are available to look into it.
As always, if the performance of Harbour is an issue for you now and you want to see Harbour perform as per your expectations then contributions speak louder than benchmark results.