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 :

ld a,1

out ($20),a ;set 15MHz

xor a

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 !