There should be a downscale mirror mode in Displays for HiDPI laptops

Both OSX and Windows 10 with HiDPI laptops default to downscaled mirror for LoDPI external monitors/projectors.

Xrandrs is also able to do that, but you cannot do this with Displays preferences GUI in Ubuntu.

Currently Displays offers only the lowest common resolutions in mirror mode, but it should also offer the scaled mode for the external screen or maybe even both options: 'Scale external' and 'Scale built-in'.

This does the trick for my laptop:
xrandr --output eDP-1 --auto --output DP-1 --auto --same-as eDP-1 --scale-from 3200x1800

(assuming eDP-1 is built-in display and DP-1 is external lower-dpi screen, which is almost always the case for HiDPI laptops nowadays)