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

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