Even if you're using a newish computer, you might notice that Google Mail is waaaaay slow. Likewise other web applications. This is because they're basically big graphical programs written in Javascript.
Fortunately, Google provides a mobile version of their site for smart phones, which has most of the functionality and much less Javascript. It's designed for smaller screens, and hides more stuff inside menus, but is much less sluggish.
The cool thing is, there are Firefox and Chrome extensions that let you change the user-agent string. If you change it to match an Android phone, Google assumes you are in fact on a phone, and gives you the mobile version of the website... Thus, no more lag. I'll leave you to decide whether the usability impact is worth it.
... Oh, and I also noticed that GMail is broken for at least some iPhone user-agent strings; the inbox won't let you scroll down past the first page of emails. The cynic in me suspects that this is deliberate.