alt_read_flash too slow?

How big is your firmware being loaded at boot time? (I am guessing it is not close to 256KB).

For example, if it is only 1/5th of 256KB, then you are seeing roughly equivalent performance and the answer to your question is "yes, it sounds about right".

If 512KB/s is too slow and you want to make it faster, you would need to supply more information about your system / application.