I have tried many settings to improve the installation time.
I tried <wix# samples>/Custom_UI/EmbeddedUI sample with my files, and I got the same installation time for attended and unattended modes (the longer time).
At some point, I tried to remove the embedded cabinet file from MSI file and bingo, the clean installation time dropped off from 7 minutes to 2 minutes. So it seems that not embedding the cabinet file is a solution for me for UI installations to run faster.
I tried <wix# samples>/Custom_UI/EmbeddedUI sample with my files, and I got the same installation time for attended and unattended modes (the longer time).
At some point, I tried to remove the embedded cabinet file from MSI file and bingo, the clean installation time dropped off from 7 minutes to 2 minutes. So it seems that not embedding the cabinet file is a solution for me for UI installations to run faster.