I have a KORG MicroKey Air bluetooth keyboard. It works fine with REAPER (enabled for input and control with default settings) but I've noticed that if I don't have it turned on, there's a lag of several seconds between pressing play and playback actually starting. As soon as I turn the bluetooth keyboard back on, the lag disappears.
I don't always need the keyboard so I have to go into the MIDI device settings and disable it when I'm not using it.
REAPER x64 6.03, Win10