The ti 84 plus and ti 84 plus se have the same CPU speed : 15MHz
There's also a compatibility mode (6Mhz) that you can enable or disable in assembly code :
out ($20),a ;set 15MHz
out ($20),a ;set 6MHz
The differences between ti 84 plus and ti 84 plus SE is memory (archive memory not RAM).
The ti 84 plus se is fully compatible with the ti 84 plus be !