How can I "lazily" read output from xrandr?

You can use awk to remove some pipelines (processes) and to only read the file until encountering the first instance of Brightness:

xrandr --verbose | awk '/Brightness/ { print $2; exit }'

In addition to @LatinSuD's suggestion of using grep's -m flag to stop reading after a match, you can adjust the size of xrandr's stdout buffer with a tool like stdbuf like so:

BRIGHTNESS=`stdbuf -o0 xrandr --verbose | grep -m 1 -i brightness | cut -f2 -d ' '`

This can give you a significant speed increase:

$ cat brightness
xrandr --verbose | grep -m 1 -i brightness | cut -f2 -d ' '

$ time sh brightness > /dev/null
sh brightness > /dev/null  0.00s user 0.00s system 1% cpu 0.485 total

$ cat brightness_nobuffer
stdbuf -o0 xrandr --verbose | grep -m 1 -i brightness | cut -f2 -d ' '

[ para ~ . ]$ time sh brightness_nobuffer > /dev/null
sh brightness_nobuffer > /dev/null  0.01s user 0.01s system 10% cpu 0.130 total

The slowest step is: xrandr --verbose. So if you didn't have to fetch brightness using xrandr --verbose every time you try adjusting your brightness, the problem will be solved.

This can be achieved by the following workflow:

1. Export the current brightness one-time (preferably, at start-up) using: echo `xrandr --verbose | grep -m 1 -i brightness | cut -f2 -d ' '` > brightness.txt
2. And every time you need the brightness read it from brightness.txt
3. And every time you update the brightness also update the value at brightness.txt

For example this is how I update my brightness:

#!/bin/bash

# BRIGHT=`xrandr --verbose | grep -m 1 -i brightness | cut -f2 -d ' '`

BRIGHT=`cat brightness.txt`

if [ "$1" = '+' ]; then
    NEWBRIGHT=$(echo "$BRIGHT + 0.05" | bc)
    if [ "$(echo "$NEWBRIGHT > 1.0" | bc)" -eq 1 ]; then
        NEWBRIGHT='1.0'
    fi
elif [ "$1" = '-' ]; then
    NEWBRIGHT=$(echo "$BRIGHT - 0.05" | bc)
    if [ "$(echo "$NEWBRIGHT < 0.0" | bc)" -eq 1 ]; then
        NEWBRIGHT='0.0'
    fi
fi

xrandr --output eDP-1 --brightness "$NEWBRIGHT"
echo "${NEWBRIGHT}" > brightness.txt

I suggest you install the xbacklight program.

Of course, if your hardware does not support it, this is not an option. So you must resort to the software adjustment as you have been doing:

man xrandr 2>/dev/null |
grep '^ *--brightness' -A8
   --brightness brightness
          Multiply the gamma values on the  crtc  cur‐
          rently  attached  to the output to specified
          floating value. Useful for overly bright  or
          overly  dim  outputs.   However,  this  is a
          software only modification, if your hardware
          has  support  to actually change the bright‐
          ness, you will probably prefer to use xback‐
          light.

It would appear that the brightness settting is nothing more than a multiplier for the RGB gamma values of your display. As such, it might make more sense - or, at least, it may be easier - if you instead directly affected that with xgamma.

xgamma
-> Red  1.000, Green  1.000, Blue  1.000

xgamma -gamma .7
-> Red  1.000, Green  1.000, Blue  1.000
<- Red  0.700, Green  0.700, Blue  0.700

In the same vein as many other answers you can quit the input with sed as soon as you encounter a Brightness string like:

xrandr --verbose | sed '/Br/!d;s/.* //;q'

That deletes all lines before the first occurrence of the Br string. When it finds one it then removes everything up to and including the last <space> from the line before immediately quitting the input. So all that remains in a case of 100% brightness is, for instance:

echo "<$(xrandr --verbose | sed '/Br/!d;s/.* //;q')>"
<1.0>

It would be far better though if you could get a valid EDID on your output and directly affect its backlight with the following tool:

man xbacklight 2>/dev/null | 
sed '/^ *SYNOPSIS/,/^ *-inc/!d;//c\\'

       xbacklight [-help]  [-display display] [-get] [-set
       percent] [-inc percent] [-dec percent]

DESCRIPTION
       Xbacklight  is used to adjust the backlight bright‐
       ness where supported. It finds all outputs on the X
       server  supporting backlight brightness control and
       changes them all in the same way.

       -get   Print out the current  backlight  brightness
              of  each  output  with  such  a control. The
              brightness is represented as a percentage of
              the maximum brightness supported.

       -set percent
              Sets each backlight brightness to the speci‐
              fied level.

It is apparently already installed on my machine because, at some point I installed...

pacman -Qo /usr/bin/xbacklight
/usr/bin/xbacklight is owned by xorg-xbacklight 1.2.1-1

...^that package.

As xbacklight was not working for me I adapt previous answer to make a script which could be called as keyboard shortcut :

#!/bin/bash
bright=$(xrandr --verbose | awk '/Brightness/ { print $2; exit }')
case $1 in
up)
xrandr --output eDP-1 --brightness $(echo $bright+0.1 |bc -l)
;;
down)
xrandr --output eDP-1 --brightness $(echo $bright-0.1 |bc -l)
;;
*)
echo error
exit 1
;;
esac