I'm having a new 8 TB hard drive and I would like to encrypt it. I know that encrypting the full space would take hours or days.
BitLocker is Full Disk Encryption (FDE). By default if you encrypt a particular partition the entire partition is encrypted. You can continue to use your system while the encryption process happens.
I'm considering to encrypt the used space only.
You can select the less secure option to only encrypt the used space on the partition.
BitLocker in Windows 10 lets users choose to encrypt just their data. Although it's not the most secure way to encrypt a drive, this option can reduce encryption time by more than 99 percent, depending on how much data that needs to be encrypted.
Source: What is Used Disk Space Only encryption?
How secure is that and what would you suggest me if there is a better option or alternative?
Choosing to only encrypt the used space on the partition is indeed less secure than encrypting the entire partition. Only you can decide if BitLocker satisfies your security requirements. There is only one alternative to BitLocker that is actually supported and actively developed.