We care a lot about performance. One of the tools we use to make sure Web Mail is working well for you is a set of monitoring agents that constantly run from 10 cities all around the United States. The "agent" is basically an automated script that runs on a computer connected to the Internet, and opens the Internet Explorer browser to load and use Web Mail just like a real user. So this isn't a simulation, it is doing exactly what real users do on their own computers, and using the most popular software used by real users. These monitoring agents are connected to one of several different network backbones to represent a good cross section of users in different areas without skewing the results with faster or slower network paths to our servers.
Ten times every hour the script opens Internet Explorer and runs through twelve steps performing common tasks in Web Mail. It begins by loading the Web Mail login page and logs in using a regular account just like yours. Then it proceeds to refresh the Inbox view, open a message, delete a message, compose and send a message, and all the basic tasks that you do in Web Mail. And unlike your own browser settings, this test browser has the cache turned off, so it gets no speed advantage from loading locally cached files like you do. Everything loads fresh from our server on every test.
I pulled up some graphs of the results for the last 24 hours at the time I was writing this. You can see the steps in the script below, and the average times for the 24 hour period. These times are measured from when the link in the browser is clicked, until the task completes and the resulting page finishes loading.

Dec. 15 4:34 PM EST - Dec. 16 4:34 PM EST (average times)
These results are probably not what you will see at home, unless your computer is plugged in directly to the nearest Internet backbone node like these agents are, and you have a top-speed computer. So we aren't measuring the performance of your home connection here, but we are measuring the performance of Web Mail. On my own cable modem connection at home, when I run the same script I usually get results in the same range as those shown. But it is probably a little slower on average. Note also that the script doesn't wait for advertisements to load (and you don't have to either). It clicks the next task as soon as the Web Mail page has finished reloading. And things like sending messages do depend on the size of the content; the script sends a one-word email on that step (the message says "Testing"). Robots can type pretty fast too.
Here's another graph showing all the tests runs during the same period (this past day) as shown in the task list above. This one shows the total time for all twelve steps to complete. These times are one of the things we watch carefully, and if we see a test that has an unusually high time, we can drill down using the information in the top chart above (and a lot of other detail we get) to find the problem.

Total Script Time
The charts above show that Web Mail is working fine today. The different times in the graph are showing that the different cities and networks we measure from have slightly different performance (see how they repeat every hour), but they are very consistent today and it looks like all is well. This test is only measuring the Web Mail pages and checking and sending email, it isn't measuring some of the other related systems like setting Preferences. We have other monitoring tools to do that.
So these end-to-end tests of our Web Mail system are one way we are working to keep everything in top shape for you. When problems show up from these monitoring tools and others that we use, alarms go off and a team of people acts to get it fixed as quickly as they can. Of course the results aren't always as consistent and nice looking as these, but we do work hard to keep it that way.