Bad performance bash, changing display brightness
I'm confused about your goal for this question:


performance is bad
performance isn't terrible
improvements would be appreciated
improvements have nothing to do with performance.


Nevertheless, code review notes:


you don't have to export every variable, only the ones that are needed by processes you spawn from this script.
validate inputs: why do anything if the input is neither "0" nor "1"?


Here's my take:

#!/bin/bash                                                                                      
# increase/decrease display brightness of xrandr of both displays                                
# use:                                                                                           
# $ ./change_brightness [up/down (0/1)]                                                          

current_brightness=$(xrandr --verbose | awk '/Brightness/ { print $2; exit }')            
case $1 in
    0) direction=-1;;
    1) direction=1;;
    *) echo "some error message"; exit 1;;
esac

export current_brightness direction
new_brightness=$( perl -E '
    use List::Util qw( min max );
    say max(0, min($ENV{current_brightness} + $ENV{direction} * 0.2, 1.0));
')

xrandr --output eDP-1-1 --brightness $new_brightness                                             
xrandr --output DP-0 --brightness $new_brightness