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.