Bluetooth low energy paired vs unpaired communication
If you need security, you need pairing and bonding. 

There is another term called "bonding" in BLE, which might be exactly what you want. Let me give you a short recap...

Pairing
This is the process of key generation and setting up a secure communication for a connection. Of course, this involves some communication overhead (exchanging public keys, generating a common symmetric long term key, ...). You won't do that EVERY time you connect devices but just once.  

Bonding
After pairing, it is important to STORE the generated keys from the pairing process. This is called bonding. It enables two devices to securely communicate with each other with a key that was generated during the pairing process. After bonding, communication between devices has no overhead other than the cypher.  

Just Works
This is basically the input/output capabilities of your device. Consider a device without buttons or display. The only way to setup a secure communication is "just works", because there is no possibility to have additional passwords or numbers that can be exchanged out-of-band (e.g. pin code entry or NFC).

One other thing: In BLE, there is always "legacy" security and since version 4.2 the newer "LESC" security. LESC is the only one that is considered secure today.