TL;DR: Don't worry, the CPU temps shown in the BIOS are high because the CPU in the BIOS runs at its full clock speed
On my PC, I'm seeing ~50 degrees celsius in the BIOS / UEFI, and just 25 degrees celsius while idling on Windows as shown by Real Temp.
Here is why: In the BIOS, the CPU is running at its full clock speed [so ~4GHz on my i7 6700K]. When idling on Windows however, the CPU clock goes all the way down to just 800MHz. At this reduced frequency, the CPU consumes much less power, and also generates much less heat.
Then, running a CPU stress test automatically gets the CPU to increase its clock back to the ~4GHz, and I'm then getting temperatures of about 50 to 55 degrees celsius at 100% load.
So, don't panic because you're seeing high temps in the BIOS.
Note: Read about Intel's SpeedStep feature, which changes the CPU's clock speed:
https://en.wikipedia.org/wiki/SpeedStep
Note: A CPU can idle even when its clock runs at 4GHz - this will happen if you disable the SpeedStep feature for instance