Completely backward compatible with Bluetooth 4.0, 4.1, 4.2.
By using Coded physical layers (PHYs), receiver sensitivity improves while maintaining the same power draw as in BT 4.0, 4.1., 4.2. BT 5 uses forward error correction (FEC) to expand payload data bits 1-to-2 or 1-to-4 at data rates of 500 Kbps or 125 Kbps.
Both high speed and long range is not possible at the same time. Devices can either operate at 2 Mbps at a lower range, or at longer ranges with a lower data rate (to achieve maximum range, it needs to be dropped to 125 Kbps).
BT 5 does NOT improve the delay between packets in comparison to BT 4. It just increases the packet size. This is why the power draw does not increase in comparison to BT 4.
To make use of BT5 high speed or long range, both the central and the peripheral need to have BT5 support. For example, a long-range sensors device will not be able to communicate with a phone that does not support BT 5 Coded PHYs.