The windows experience index is largely irrelevant,its not an accurate score of real-time performance but more of a shitty number scale to compare hardware(We have better scales here on Toms!). But it is "correct" in this case,your 745 is a weak card and the bottleneck of your system.
I just ran the WEI for comparisons sake and my R7 260x got 7.7 and its a <$150 card.
It's the best value you can get at this time,aswell.
EDIT: and if you want a numbers comparison; the 260x is 2-3 times more powerful than your 745.