transferring songs from computer to iphone through iTunes

hello i've got two sets of albums to transfer from computer to iphone through iTune, In iTune, I go to my device and the Music, then I drag and drop the music from computer folders to the iTune layout,

There are some strange things happening during the process. When I transfer the first album , there were about 10 songs in mp3 format, when I did the drag and drop and it took a very very long time to finish copying, and during the time my iPhone suddenly gets hot on its case, then after finishing this one I do the same with another album immediately, having the similar number of songs and size in total but it doesn't get hot anymore, feels normal.

I would like to know

1. if drag and drop is a technically correct way to transfer songs?
2. why such difference happen (time and temp on the case) but the two sets of albums are in similar sizes and same formats.

Thank you very much