DELL Latitude E7250 slow BIOS boot/POST time

My DELL E7250 is having the problem with BIOS POST time. My unit has an 128GB SATA 3 SSD. The problem is that the laptop when has BIOS boot always takes up to 15 second. However when booting into Windows it takes for only 5 second to reach to the login screen. I tried to choose Fast Boot into Minimal, Extended BIOS delay to 0 second and even reflashing the A24 BIOS version but the problem still persists. I see the XPS 13 9343 with the same configuration (8GB RAM, 128 GB SATA 3 SSD and i5-5300U) does the BIOS POST in 5 second. Sometimes my machine even does BIOS boot only in 5 seconds but usually it took 15 sec to do. Tried in UEFI mode and Legacy mode, NO DIFFERENCE. Can all of you please give me the solution or at least report the same problem in this discussion with any DELL model with 5th generation Broadwell chip? Appreciate to have the solution for this problem as soon as possible.