kimsiang85:
You should know that a microcontroller is not a serial access device like a cassette tape. You can easily fill the entire code space by placing all your code at the end of ROM and filling the upper locations with 0's, F's and such. Just because code is at the end of address space does not make it slow because it can be accessed is the same time as the very first byte of code, a.k.a. random access.
What impacts the PIC speed is the techniques of excessively using macros, calling tables, and moving bulks of RAM using hard coded labels as opposed to indirect addressing, and placing faith in compilers.