I'm interested to know what effect on performance it has, to repartition and format the eMMC.

My understanding is that the eMMC works like a memory card. A while ago I tried formatting one to ext3, it worked at approximately half the original speed. Then when I formatted it on a PC back to FAT32, it was still about half the original speed. It was only when I used the manufacturers flash formatting software did it go back to full speed.

The clusters of sectors in the filesystem should be aligned with the physical blocks of the flash memory. Otherwise it results in multiple accesses, slowing everything down.

The flash formatting software put the start of partition at an offset from the start of the card memory, and had a different size for the FAT data when compared with the Windows partitioning & formatting.

It is necessary to get both the address of the start of the partition, and also the address of the first cluster of data (which follows the metadata) correct, to get the full performance.

This is a link I found to an explanation of cluster alignment:

http://www.hjreggel.net/cardspeed/cs_calign.html