@loyukfai: thanks a lot for all the detail
We are aware of the current slowdowns that can occure when using selection/highlight, and it is one of the enhancements that are high on our TODO list.
Lastly, please consider adding more layout customization for EPUB, like background colour, font type, size and line spacing.
We have worked hard on the EPUB customization this week, and I'm pleased to tell you that the next release will be a major up on that side.
Text reflow for PDF could be useful too.
Interestingly, this feature has been present since day 1. You can activate text reflow from the "display settings" button when viewing a PDF. I guess this shows that there is still room for improvement