Hopefully, this tip may help you out and improve your WiFi speed when Bluetooth is turned on. Now let's get on with it.
1. Go to your Notification Shade and Long press the 'GPS' shortcut icon ( which brings you to the Location settings).
2. Tap on 'Scanning'.
3. Disable Bluetooth scanning.
Previously, it maxed out at about 1.5 to 1.6MB/s when bluetooth is turned on. After turning off 'Bluetooth scanning', WiFi speed could max out at about 3.6MB/s or 28.8Mbps. Result does fluctuate to a low 1.8MB/s and goes back up, but never unusable WiFi speeds.
Remember, Redmi Note 8 has only 1x1 MIMO ( single stream client ) WiFi. Do not compare it to another device which has 2x2 MIMO ( two streams client ) WiFi. Even the Redmi Note 6 Pro has 2x2 MIMO.