Delay in recognizing bluetooth keyboard & mouse

Problem:

I have fresh installed High Sierra with all updates. At the login screen I am experiencing about a 5-10 second delay before the bluetooth keyboard & mouse are recognized and I'm able to select user to login and enter password. The issue is reproducible since first boot after clear install.

Device:

iMac (Retina 5K, 27-inch, 2017)

3.8 GHz Intel Core i5

8 GB 2400 MHz DDR4

Radeon Pro 580 8 GB

OS:

macOS High Sierra 10.13 (17A365)