After I updated my LibreOffice from 5.2.x 32-bit to 5.3.x 64-bit Calc has become sluggish. Moving between cells by pressing Ctrl + arrow keys to jump to the next populated cell is not instantaneous like before. Now, it takes about half a second. The same goes when I click to change tab in the same document, or scroll up or down the document. All these operations were very quick in 5.2.x 32-bit. 

Another thing I noticed is that the fonts have changed somehow, although I cannot describe exactly what it is. It is as if they are "pointier", with pixels protruding instead of having smooth lines. This make the smaller fonts a bit less readable.