This uses PhantomJS and YSlow for PhantomJS so get the min/max/average page load time for a specific url over a number of iterations. This script assumes you have PhantomJS installed and will download yslow if it's not in the correct place.

perf_yslow.sh

#!/bin/bash

# Example usage: sh perf_yslow.sh -l 3 -u http://example.com

while getopts "l:u:" opt; do
  case $opt in
    l)
      LOOPS=$OPTARG
      ;;
    u)
      URL=$OPTARG
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
  esac
done

# Check PhantomJS is installed
command -v phantomjs >/dev/null 2>&1 || { echo >&2 "Phantomjs is not installed.  Aborting."; exit 1; }

# Check we have yslow
if [ ! -f yslow.js ]; then
   echo "ySlow not found, downloading"
   wget http://yslow.org/yslow-phantomjs-3.1.8.zip
   unzip yslow-phantomjs-3.1.8.zip
fi

echo "###############################"
echo "# Testing: ${URL}"
echo "# Loops: ${LOOPS}"
echo "###############################"

rm -rf load_times.txt

# Loop through all the tables and export
for (( c=1; c<=LOOPS; c++ ))
do
  echo "Running test ${c} of ${LOOPS}"
  # run the test, get the page load line, get the time, trim whitespace and add to file
  phantomjs yslow.js --info basic --format plain ${URL} |
    grep 'page load time:' |
    cut -d: -f 2 |
    tr -d ' ' >> load_times.txt
done

# find the min / max / average
# from: http://unix.stackexchange.com/a/13738/30901
INFO=$(cat load_times.txt | awk 'NR == 1 { max=$1; min=$1; sum=0 }
      { if ($1>max) max=$1; if ($1<min) min=$1; sum+=$1;}
      END {printf "Min: %d\tMax: %d\tAverage: %f\n", min, max, sum/NR}')

# add the median
INFO="${INFO}\tMedian: $(cat load_times.txt | sort -n | awk '{arr[NR]=$1}
   END { if (NR%2==1) print arr[(NR+1)/2]; else print (arr[NR/2]+arr[NR/2+1])/2}')"

echo ${INFO}