R: Add interpolated values in between columns of dataframe?

I have a data frame that looks like this

Region      2000    2001   2002    2003    2004      2005
Australia   15.6    18.4   19.2    20.2    39.1      50.2
Norway      19.05   20.2   15.3    10      10.1      5.6

and basically I need a quick way to add extra columns in-between the currently existing columns that contain interpolated values of the surrounding columns.

Think of it like this: say you don't want columns for every year, but rather columns for every quarter. Then, for every pair of years (like 2000 and 2001), we would need to add 3 extra columns in-between these years.

The values of these columns will just be interpolated values. So, for Australia, the value in 2000 is 15.6 and in 2001 it is 18.4. So we calculate (18.4 - 15.6)/4 = 0.7, and then the values should now be 15.6, 16.3, 17, 17.7, and finally 18.4.

I have a working solution that builds up the new dataframe from scratch using a for loop. It is EXTREMELY slow. How to speed this up?