Very slow copy and paste
This one has been plaguing me too, but only intermittently - bizarrely in one file some sheets were fast, others slow.
What I discovered, by chance, was that there were hidden objects sitting on a layer above the cells - revealed by accident only if I moved the cursor slowly around a sheet, and cross occasionally turned either to an arrow, or the floating text cursor. Using the shift key I could hunt these down in a cluster, then delete, but it literally was like playing 'pin the tail on the donkey'.
This process would delete most objects, but for some reason a long column would always remain. Each deletion just seemed to spawn a new 'child'. Haven't a clue what was causing this, but it was frustrating.
Like others on various fora I have been frustrated by the lack of an ability to select / lasso a area to highlight anything that might be lurking; then, after a lot of searching, I found this site:
http://www.excelforum.com/excel-new-users-basics/761143-ghostly-text-boxes.html
...and this advice.
Hit ctrl+g
Select Special
Select objects
Any objects on your worksheet are then highlighted - you can then delete.
This seems to kill everything, including the pillar of objects that keep reforming, and the copy + paste on previously affected sheets is now back to normal.
Hurrah!