The macros and functions used to retrieve data from the Program Space have to generate some extra code in order to actually load the data from the Program Space. This incurs some extra overhead in terms of code space (extra opcodes) and execution time. Usually, both the space and time overhead is minimal compared to the space savings of putting data in Program Space. But you should be aware of this so you can minimize the number of calls within a single function that gets the same piece of data from Program Space. It is always instructive to look at the resulting disassembly from the compiler.