I would suggest trying to make sure that you are using the categories efficiently; one category with > 50k images isn't very efficient. We are looking at ways to improve some of our queries for larger sites, but PhotoPost was designed with many features for the large majority of our customers; larger sites, to be more efficent, need less options.
For example, we could easily reduce the size of queries if we didn't allow categories to be sorted - just limiting the display to the photo date would allow us to limit our queries based on the date of the photo being displayed - but when you allow the flexibility of sorting a gallery and then having that sort "stick" while viewing photos, well, then its going to use more resources.
It's not that PhotoPost can't scale to large systems, it's that you need to manage your options accordingly and possibly even look at pulling back on some of the options to allow even greater scalability.
Please do not PM me for support or sales questions. Thank you for your understanding.