The Onyx devices still present problems for KOReader, I guess, there should not be those performance issues. Things are quite snappy on my old ASUS TF-101 tablet.
KOReader uses the Coolreader 3 Engine, to read epub, if your device is having problems with Coolreader, the issues could be related.
You can tweak the top, bottom, and side margins, but you should keep in mind the line height might be just enough that another line wouldn't properly fit. Try using a slightly larger or smaller font size, and/or adjusting the line height.
You can have a number of dictionaries, for the most part they are formatted as
shown in the wiki.
Luck;
Ken