If I want to heat lets say, 1 liter of water - would it be faster if I heat half a liter, and then another half a liter? or slower? my guess is that its not equal (lets assume that the time it takes for me to switch the water in the pot is 0)
Heating by 2 parts will be slower to heating the whole volume, but will provide part of the volume heated faster.
Even if we ideally consider replacement delay as zero, there are extra heat loses by manipulation with heated water and it's cooling while the other part is being heated.
Therefore heating to the same final temperature takes longer, if heated by parts.