So, I was having this issue after windows 8.1 up until about five seconds ago. For my Laptop, however, I have an AMD-yada-yada Display Adapter, and when I tried to update it - in hopes that would fix the issue - it said I was already fully updated and the issue persisted. However, I decided to simply disable then re-enable it, to see if that would do anything, and voila, slow right-click menu is now instant on Desktop. (Also, since it took me a while to find everything - since everyone else here sorta expected you knew where device manager was - I decided I'd make a steps to reach it / try this fix guide...)
1. Open side menu
2. Click Settings
3. Click PC Info
4. On the left of PC Info window, click Device Manager.
5. In the Display Adapters section, right-click on your adapter, and click Disable. (Screen will flash off temporarily.)
6. Right Click on the now-disabled Display Adapter, and click Enable. (Screen will flash off again, but your slow right click should be fixed.)
Now I'm wondering if this also had to do with my computer randomly changing brightness/saturation...
Since that was changing erratically during use after 8.1 as well... I suppose it could be it, if the display adapter manages those things at all... Anyway, since there seems to be some variability in information, hope this helps!