If you want to monitor a website you can setup easily a regular cron to check the performances.

You just need PhantomJS which is a headless browser. For a 64bits version:

if ! which bzip2 >/dev/null; then
    apt-get install bzip2
fi
if ! [ -f "/usr/local/bin/phantomjs" ]; then
    cd /tmp/
    wget https://phantomjs.googlecode.com/files/phantomjs-1.9.2-linux-i686.tar.bz2
    tar xvf phantomjs-1.9.2-linux-i686.tar.bz2
    cp phantomjs-1.9.2-linux-i686/bin/phantomjs /usr/local/bin/
    chmod +x /usr/local/bin/phantomjs
fi

Then we will use confess.js script with PhantomJS. Download the confess.js.

Create a basic configuration file config.json:

{
  "task": "appcache",
  "userAgent": "chrome",
  "userAgentAliases": {
    "iphone": "Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7",
    "android": "Mozilla/5.0 (Linux; U; Android 2.2; en-us; Nexus One Build/FRF91) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1",
    "chrome": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.12 Safari/535.11"
  },
  "wait": 0,
  "consolePrefix": "#",
  "verbose": true,
  "appcache": {
    "urlsFromDocument": true,
    "urlsFromRequests": false,
    "cacheFilter": ".*",
    "networkFilter": null
  }
}

Then you can launch phantomJS:

/usr/local/bin/phantomjs confess.js http://www.google.com/ performance config.json

You can combine this to receive a mail alert when loading time rise above a limit:

if ! which mail >/dev/null; then
    apt-get install mailutils
fi

/usr/local/bin/phantomjs confess.js http://www.google.com/ performance config.json > daily-report.txt

TOTAL=`cat daily-report.txt|grep 'Elapsed load time:'|sed 's/Elapsed load time:[ ]*//g'|sed 's/ms//g'`
if [ $TOTAL -gt 8000 ];then
    mail -s "[ALERT][Monitoring] - Loading time $TOTAL ms" $MAIL < daily-report.txt
fi

rm daily-report.txt

Add all in a script, create a cron and you have a basic monitoring system.