Follow this order when composing emails:
* First think if you have to attach files to your email. Select the attachments so they will be already uploaded by the time you finish your mail. If you have many small files to attach or text documents, put all the files in an archive. It's faster and easier to download.
* Then type the text of your email. For compatibility reasons, stay with plain text. Most of the time HTML emails will create problems or will be useless.
* Check the spelling of your text. Gmail has a very cool spell checker with support for 38 languages.
* Then summarize the content of your email and type a meaningful title ("Check this!" or "Hi" aren't good examples of meaningful titles).
* Insert the recipient's email address. If you have more recipients, don't forget to include them in the BCC section.
* Send your mail. This should happen instantly.
Inspired by Google Blogoscoped.
Also see:
5 fast ways to check Gmail