The data transfer speed of devices connected via Bluetooth varies depending on the Bluetooth version in use:

• Bluetooth 4.0: Up to 25 Mbps (theoretical maximum) with Bluetooth Low Energy (BLE).
• Bluetooth 4.1: Similar speeds as Bluetooth 4.0 but with improvements in data transmission and multiplexing.
• Bluetooth 4.2: Up to 1 Mbps for BLE devices, with enhancements for privacy.
• Bluetooth 5.0: Up to 2 Mbps for BLE, and allows for improved range and more data capacity.
• Bluetooth 5.1 and newer: Continued enhancements in speed, range, and connection stability.

However, practical speeds are often lower due to factors such as distance, interference, and device capabilities.