In defense of eReader
While it can take a bit longer to open ebooks, it shouldn't be a "wet week"
Maybe there's something odd about the Tom Sawyer version that you were trying (illustrations?). But even so, once loaded, eReader is just as fast as Stanza.
No questions, that changing the font size takes time to ripple the changes things through, and it's worse than Stanza by far in that regard. But, realistically, how often does one do that? Typically, figuring which font, size, and margin settings works best is done only once! Once you have the preferred set up, eReader works just as well as Stanza. Indeed, it's dictionary look up feature is better than Stanza's.
In general, eReader holds its own vis-a-vis Stanza. (Although being older, it's less multi-task aware and doesn't instantly return to the same screen you left off. It briefly flashes the load screen, but much faster than a regular open.)
But if Calibre can do a convenient mass .pdb to epub, go for it!
I just like having two ebook readers in operation as it makes it easier to switch between books than switching books within one reader.