Y!Slow has some basic figures, but to really figure it out you need to decide what you want to optimise. Are you concerned about getting something to be shown in the browser (generation + transport + rendering), time taken for the server to process the page (generation), time taken for data transfer (transport)...
Try to measure the specific thing you want to improve upon, so then you can see the difference most reliably. If you want to improve transport time, then pull a static copy of the page (almost zero processing time) and download it with an app such as wget rather than view it in a browser.
For page processing time, build in timing to the page either directly into the programming or into the apache configuration.
Page render time can be tricky. I have done it in the past using a local copy of the site and a virtual machine with far too few resources. Make a virtual machine, boot the browser, clear the cache, scale back CPU to 1% and RAM to 1Mb and time loading the page from the local drive.