If you discover a way to make the system 100X faster, and the reason is someone else's code, then tell that programmer, and only them, and tell them gently what you found and leave it with them to be the person who implements it, or offer to implement a solution together if they want to.
They may or may not take sole credit but it doesn't matter. Definitely do not sneakily try to let people know it was really you who is responsible.
If you discover a way to make the system 100X faster, and the reason is someone else's code, then tell that programmer, and only them, and tell them gently what you found and leave it with them to be the person who implements it, or offer to implement a solution together if they want to.
They may or may not take sole credit but it doesn't matter. Definitely do not sneakily try to let people know it was really you who is responsible.