I personally like Coops' answer, but if you want an independent check of load time (ie, how long did it take me to get all the page data from the server, rather than how long did the server think it took to serve all the page data) this answer may be helpful.
If you're already running NAGIOS or ICINGA, the
check_http plugin can evaluate loading times and go WARNING or CRITICAL if they exceed certain thresholds. I'm not sure I'd put either of them in just for that, but if you're already using one, the plugin's ideally suited to qualitative monitoring (how often did this page take too long to load?).
If you'd prefer quantitative data (how long does this page take to load?) and are running munin, the check_http_requisites plugin will track that.
Many other monitoring systems will no doubt have similar functionality; what are you doing your internal monitoring with at the moment?
And if the answer above is "nothing", I beg your forgiveness if I go all soapbox for a moment: now is the time to put a proper monitoring solution in. I know it's tempting to pop in just-another-one-line-script, but they rapidly grow to be unmaintainable. Whereas once you're put in a good, free, plugin-based monitoring solution you suddenly discover all sorts of other things it would be really useful to monitor, and you don't have to reinvent the data-gathering, data-collating, data-display, and error-notification infrastructures each time.