Originally Posted by GeoffR
To check whether the problem is with sideloaded books in general or something wrong with the specific books you have sideloaded, you could download the epub version of the free Kobo book that had no problems and see if there is any difference when it is sideloaded.
I had problems with a book I bought from Kobo where there was one giant html file for the whole book. I didn't read it long enough to see if it caused the battery to drain, but it was very slow to turn pages. Kobo offered me store credit, but I decided to keep the book because all the problems disappeared once I ran the epub version through Calibre to break up the html file into one file per chapter.
That is something I could try. However, 2 of the books that I tried with the Glo function perfectly well on other devices.