This is a very good question since to me fast access to books and fast navigation are very important questions, and influence a lot my ereading choices
Assuming that the book is on the memory card and I have not changed the content of the memory card (else several seconds add when Ebookwise recognizes the new titles):
turn on 1s
open book 1s, opens at last page read if any, can switch instantaneously between 2 titles with a button, otherwise instantaneous access to library by finger/stylus touching
navigation: instantaneous page turning, paginated with numbers, very fast moving between pages by moving the page indicator with the stylus or finger, bookmarkable, clickable chapters if book so prepared.
My ereader of choice for short story collections, magazines, and for rereads where you do not want to read all pages sequentially. Also great for series where you want to move between various books fast.
I do not think any current ereader can beat the navigation capabilities of Ebookwise1150
power up 45s; however I very rarely shut it down, I just slide it in and out of cover and keep it open at the last page read, so for practical purposes turn on is 1s
open book - if the title is in the library it takes 2-10s to open it, otherwise takes another 30s-1' to find it on the card, add it and write its info to memory, choose encoding; in some formats like opf, fb2, it does this automatically
navigation - page turning is instantaneous, but navigation is not great since the page indicator is not paginated, just a bar, also because of smaller device size it is harder to move the bar accurately; navigation between books also noticeably slower, due to loading time of several seconds
My ereader of choice for new novels that I read page to page, and of course for on the go reading.