How long will initial encryption take when BitLocker is turned on?
Although BitLocker encryption occurs in the background while you continue to work, and the system remains usable, encryption times vary depending on the type of drive that is being encrypted, the size of the drive, and the speed of the drive. If you are encrypting very large drives, you may want to set encryption to occur during times when you will not be using the drive.
Sounds like the first-time encryption process is low-priority, so it will run slower if other (normal priority) programs are also using the CPU. I haven't found information about this but it might also be deliberately rate-limited to avoid interfering with normal work, which will make it run longer but cooler.
What happens if the computer is turned off during encryption or decryption?
If the computer is turned off or goes into hibernation, the BitLocker encryption and decryption process will resume where it stopped the next time Windows starts. This is true even if the power is suddenly unavailable.
So you can turn off the computer to give the SSD a break, if you feel that it's necessary.
https://harddrivegeek.com/ssd-temperature/
Most SSDs implement thermal throttling as a safety feature if a drive gets too hot. The closer your drives gets to the 70ºC limit that most manufacturers have, the more likely it is that the drive will start slowing down to prevent itself from failing. The drive will slow down drastically until the temperatures come back to a reasonable range (around 50ºC or so), at which point you should get your speeds back.
This article says that although high temperatures aren't good, the SSD itself will take actions to avoid them by monitoring its own temperature and throttling write speed (similarly to how CPUs and GPUs also have thermal throttling).
