FastStream is just the SBC codec with set parameters and the voice-back stream added, so it's bi-directional and provides better audio quality for both playback and recording than HSP (Handset Profile) with CVSD codec.

Check out this link for more information.

Codec details
Bit Depth
Max Frequency
48 kHz
Max Bitrate
212 kbps
Audio delay (latency)*
~ 40ms
Audio latency
~ 30-40 ms