Reading should never be a test of stamina.
If a book is slow, becoming a chore to read, and beginning to lose your attention; put it down and move on to something else.
You should never be afraid to leave a book unfinished, nor disappointed that it came to that.
I know the feeling of wanting to stick with a book; the obligation you feel to keep going, with the hope that it may pick up pace, or change tack, or provide invaluable information you would have otherwise missed if you had stopped sooner.
The truth is: you don't owe the author your valuable time.
Set it to one side and move on. Remember: you can always return to a book at a later date.