How to identify why site is slow

Hi,

I built mostly small drupal 7 sites with basic functionality in most of them, including views and sometimes some image slider modules such as MD Slider.

These are hosted by a good host on shared hosting servers.

Usually I notice good speeds and often turn on css and js aggregation under the site configuration/performance setting.

This is usually sufficient.

A recent site is running a few seconds slower than others.

How do I identify where and why it is running slow.

My hosting provider suggests turning off all modules and turning them back on one at at time to try and see where the problem might lie.

Here is part of their reply:

"I can see that the delay is around 1 second "time to first byte" which is while the site processes MySQL queries, so it seems there is something causing complex/long queries for this site hence it feels sluggish.

It may be worth turning off any modules and seeing how the site feels, then enabling them one at a time incase there is a bad/intensive module or theme?"

Is there anything a little more tangible and scientific I can do to identify any bottleneck?

Thanks,

Sam