Ajax file uploads are very slow

[eluser]Unknown[/eluser]
Hi to all.

After many hours research on the internet, I have to post a topic here because I didn't find any good answer.

I have a form with file upload.
when using this form the normal way (I mean through a normal POST query)
it's very quick and works perfectly.

but when I use an ajax form to submit.
(i'm using this library now.http://jquery.malsup.com/form/)
I used several different ajax file upload libraries.
But when I apply them, it's veeeeery slow.
and sometimes the server throws me a 500 error.

My server is a hosting server that I posess. it runs about 10 websites on it.
and it's not my computer that's too slow sending the request.

It's just when I use ajax to send this form that the request takes so long.

Any Idea about this ?

P.S. I use the last version of CI, the 1.7.2