I ask this because encrypting an hard drive using LUKS takes a few seconds while encrypting the same drive using Bitlocker takes several hours. Does that mean LUKS encryption is weaker?
If you don't select overwrite empty disk space option with LUKS, it is quick and not quite as secure. You are comparing LUKS's quick, less secure usage with Bitlocker's secure usage.
If you selected this option already: Check your disks maximum possible write speed, divide disk size to this speed. That is the minimum time overwriting can be completed. If overwriting ends quicker than this, it means there must be an error and process terminates earlier.
Example: Let's say you have 240GB SSD with maximum write speed 500MB/s.
Then minimum time is:
240GB / 500MB/s = 480s
If you do not overwrite the partition's free space, it's possible for an attacker to do two things:
Scan the partition and view unencrypted data in places where you haven't written yet.
Compare blocks that contain unencrypted data versus encrypted data and estimate how much data the volume has, or how long it has been used.
New data written to an encrypted volume will still be encrypted, and inaccessible without the key, even if you don't overwrite the free space.
You should always overwrite a partition if you don't ever want the underlying data to be accessible again, whether or not you are encrypting the new volume.