I believe it is similar to:
When you pass job interview and accept terms and conditions and being empyed to do the job, you cannot come to your boss in a few days and say: my salary is too low and I don't like doing this particular bit. He will tell you that you agreed of your own volition to do the job and perform up to certain standards and to be paid a specific salary.
If you agreed that you have to start your job at 6 am and you signed the contract, you cannot demand alter it and start at 10 am.
Sony Reader is designed in such a way that you can use LRF file with certain fonts and if they are uploaded with eBook Library the paginating is done by the powerful software+pc vs. small and weak reader
When you are trying to do something different and get issues with LRF speed, it is not LRF's fault. It is your fault that you do not follow the agreement you have made when you bought the gadget including special software as you were well aware you should do when you paid the money.
If you agreed to pay money for a gadget with optimal performance under Windows OS, you cannot now say epub is a lot faster than LRF because it is not.
P.S. btw., it is nothing personal. And I do not work for Sony
I am only saying that epub is no faster than LRF