Why is webmail so slow?

I’m talking about Gmail here, but the problem is common to almost every web mailer.

A few days ago I was sitting in a coffee shop and observed many other people sitting there while working on their laptops. Nearly everyone had a web browser open and was reading their email through an online webmail site like Hotmail or Yahoo Mail.

I then ran a little informal test: I pulled up a Yahoo Mail account and read the first 100 messages in my inbox at normal speed, without doing anything else. After about two minutes of this, the performance was extremely choppy and it took a long time to scroll from one message to another. I quit after five or ten minutes because the performance was so bad that it wasn’t worth it.

The interesting thing is that everyone I saw reading their email through webmail had exactly this experience, and no one seemed to care. The performance became very slow after just a few minutes of browsing mail, but people continued to read mail anyway.