Mail.app loads sent messages constantly

Hello, i'm writing because i don't know if that's me becoming paranoid or thats a normal mail feature. I rarely check my sent mail but today i did that and i noticed that almost any sent message i check, it loads it for about 10 seconds. I select a message and on the right there is written: Loading... and if i check the mail activity, it says Downloading message. That happens with almost every sent message i have in the sent folder. I have a fast internet connection and it loads just in 1 sec from gmail online mail. So why the **** mail.app downloads it for 10 seconds? And they are only text messages. And the strange thing that after i read the message and back to inbox, than go again into sent folder and select the same sent message, it loads again. Shouldn't they be on my HD?