Quickiest way to determine why a site is sluggish?

I just picked up a client who's Wordpress web site takes anywhere between 8 to 22 seconds to START loading. The loading delay also occurs when using the Wordpress backend so I'd like to fix the loading issue first before starting my work (template re-design). What's the quickest yet efficient way to determine why this Wordpress site is taking so long to start loading?

Thanks in advance

P.S. - They currently have a cachi开发者_运维技巧ng plugin installed (WP Super Cache) which I assume the previous web developer installed to help with the loading issue but it only helps with the front-end and not the back-end.

Try to run some test like YSlow and Google Page Speed and read their results and suggestions.

Google Speed Online is helping me a lot with analysis of my websites.

http://pagespeed.googlelabs.com/

I use browsermob. They use real browsers to test the site load performance. Shows very nice graphs showing how long each and every request took. Also shows how many requests happen in parallel. As they use real browser, you can see how long it will take to load on a real browser. Then you can choose from which location you want to test. You can choose a UK location to test how fast your page loads from UK.

By the way, I am in no way related to browsermob. I just happen to be a satisfied user of this.

And it is free.

Your server is probably loading far too many modules and is thrashing the disks as it's run out of memory.

You need to both reduce how much memory each PHP instance consumes and limit how many PHP instances can run simultanouesly to ensure you don't use virtual memory for your PHP instances.

I've written a detailed answer to a very similar problem here on Stack Overflow:

How can I figure out why my Wordpress pages load so slowly?

Well, i have came across a similar situation, such things happen when your website is hosted on a GridHosting server, which means it changes according to the server load, but sometimes the things are just opposite the scenario, the best way to check why it is slow is to first ping the website at random interval , so in this way you will know if the distance is the cause or the packet dropping is the issue, secondly, you need to make sure your server's configurations is good, i.e; request your host about a RAW log of your website, in this way you can know what is it taking long for your server to response, and the least best method is to check and make sure that your DNS resolves in a good time, and try to use some free CDN services like CloudFlare.

Hope this helps.