There's no difference between accessing an array using the bracket operators or using a pointer.

int a = foo[3];

is equivalent to

int a = *(foo + 3);

The speed hit comes from recomputing an array index instead of just incrementing; for example, writing pixels inside a double for loop. Chances are you're doing something like:

pixel[y*WIDTH + x] = val;

every iteration. Computers are very good at adding 1 -- just start the pixel pointer in the right place and then increment it each time through.