Office 15 has built in hardware graphics acceleration which can cause issues when running on a slow machine
To turn this office open any office product (Word, Excel, PowerPoint etc.) Click File -> Options -> Advanced -> Scroll down to the “Display” section -> Uncheck “Disable hardware graphics acceleration” and restart office. (*See notes below*)
This should hopefully resolve the issue with Office 2013 being slow when compared with other software.
Some people have had the issue where it is unchecked by default and checking it resolves the issue.
This issue occurs where it is unchecked but you have a graphics card that wants to accelerate the feature (The opposite to the above but causes the same issue). In this case you should check the box.