For FBReader on a Windows PC, searching for Rock (not ignoring case) was virtually instantaneous in the Plucker version of
Project Gutenberg EBook of A Tale of Two Cities, by Charles Dickens. This version is one HTML file (336 KB), but I have multi-section versions of the same e-book in Plucker format (i.e. plucked from multiple nested HTML files) and the search time was about the same.
You should check on a Desktop (Windows or Linux) that FBReader search will fit your needs. It searches within a single e-book and is a page-based search, i.e. if you search for "the" it will show you the 1st page containing the (all occurrences highlighted) and clicking next takes you to the next page containing the (not the 2nd occurrence of "the" in the document). Also, searching for "the" will find "they". I'm sure the developers would welcome suggestions for improvement (e.g. whole word only search).