Why my self-defined QSys component does not have the same speed as on-chip memory ?

Adding bursting doesn't make your IP faster, you should only add it if you need it. That code you attached doesn't look like it's going to achieve what you are looking for. You have sections in the always block that are modifying the register "bc" so if that value is decrementing for each beat of the burst and getting reloaded by the next read at the same time you will most likely have a functional failure. Instead of spending time explaining how to structure this code I suspect your IP doesn't really need to support bursting since what you appear to be trying to code looks like a counter.