Sure you already know this, but if someone don’t:
Maybe some of you have this in your *.c GObject files:
#define GET_PRIV(obj) (G_TYPE_INSTANCE_GET_PRIVATE ((obj), MY_TYPE_OBJECT, MyObjectPriv))
And then you use this macro in all your functions to access the GObject private structure.
You shouldn’t do this.
Instead, declare a *priv member in public structure and then access private members with my_object->priv->var
Maybe you think the performance improvement is not a lot. Well, I applied a similar patch to giggle  and now the startup time is ~10 second faster when opening the evolution git repository.
So, I think it’s worth
Update: From Edward Hervey comments: Some of the benefits of use of G_TYPE_INSTANCE_GET_PRIVATE(): memory space gets allocated alongside your instance, avoids an extra alloc, memory is contiguous, …