XPS 9500 Slow boot first partition / VHD

Hello !

I have installed a VHD for my professional school and for some time the loading of this VHD partition is extremely slow.

The boot interface displays correctly and quickly when I choose my private partition I boot quickly, but when I choose my VHD the Dell logo displays and grinds for 2-3 good minutes.

I have reinstalled my PC to the dell factory image and updated the BIOS 1.9.1 for my XPS 15 9500, but the slowness problem for my school partition is still slow.

Can you help me please ?

Thanks