Since ajax, a lot of people are thinking that asynchrone means “in a separated thread”. They are wrong!
The XMLHttpRequest object gives you the option to make a synchronous request to a server with the parameter async
Now you are being cool! The asynchronous is used by everyone and it doesn't freeze the page while the call to the server is done. So, if you have a call that takes 30 seconds before receiving the answer, your users will never know that you're a bad server-side coder that do not optimize the code he's writing (I'm just joking).
The misconception of asynchronous
So if you're running a lot of ajax requests simultaneously, you might get some weird behavior because they will all wait one on another before executing themselves.