There is the e-ink refresh rate which is the same (or almost the same) on all devices.
Then there is the lag between the push of the button and the start of page turning, which is due to processor speed. I know it can be annoying, but most of us have found it easy to get used to, in that we press the next page button before finishing reading the current page. I know my prs-600 is much faster (almost no lag at all), but the screen is not as clear as on other e-ink devices due to the extra layer of the touchscreen, so not ideal for manga. Ultimately, whether the lag bothers you enough or not, is a matter of personal preference.