Not sure if you've already done this but reinstall dx 9, apparently there are some files in there that aren't in dx 11 that improve performance. Also right click the .exe and launcher go into properties and set to run in XP service pack 3 mode.
I can't swear these will make much difference but a lot of people reported significant increases after doing them. If you still have a problem I'd personally reinstall windows, I'm going to guess that something that's happened over the life span of your pc has caused a slowdown somewhere, I did this on my old pc and i did get an increase so it's worth a bash especially if you have a SSD, took me like 7 minutes to install windows.
I can't swear these will make much difference but a lot of people reported significant increases after doing them. If you still have a problem I'd personally reinstall windows, I'm going to guess that something that's happened over the life span of your pc has caused a slowdown somewhere, I did this on my old pc and i did get an increase so it's worth a bash especially if you have a SSD, took me like 7 minutes to install windows.