Thanks very much for your reply and information, amachronic.
In general, Rockbox has been plenty fast enough for me so far except for extreme tasks like listing all ~11,000 tracks in my music library (which is the kind of thing I would do mostly just to see what will happen, rather than because I have a practical use/need for it).
I put the iFlash-Quad with two 128GB microSDs into a 160GB 7th Gen iPod Classic (64MB RAM) and the iFlash-Solo with one 256GB microSD (in an SD adapter) into an 80GB 5.5 Gen iPod Video (also 64MB RAM). It took the 7th Gen about 48 seconds to gather and display a list of all ~11,000 tracks, and it took the 5.5 Gen about 2min 40sec to do it with its identical copy of the same music library. Copying all of those tracks' files onto the iPods (while running the Apple firmware) and then initializing/building their databases in Rockbox was similarly much faster with the 7th Gen, though I didn't time those operations.