Vanilla Calibre: (
out of control) Comments is probably the biggest per book consumer,. IMHO Clean (edit) them of all the extra cr*p. IMHO: comments is for a synopsis. I also trim stuff like "New York Times Best Selling....

Tell me about the STORY, not the Author nor a sample chapter

(yes, there are quite a few that way)
Clean your tags. You are using Calibre. You don't need every permutation of a tag.
Calibre w/Custom Columns :
Yep they all eat more bytes.
Ask yourself if a saved search could give you what you need instead of Custom columns calculated from other stuff. (Or simply, 'Do I really need to see this' )