Yes that is completely normal for a Hard Disk Drive.
You need a Solid State drive to increase your Index.
However - an SSD will only reduce load times of all your programs and games (whatever is installed in it). It will not increase frames per second.
Does that then mean a windows experience index of 5.9 for a hard drive is a meaningless score? It is fast but it might be a lot faster than the minimum score to get a 5.9