I have a problem with chrome scrollbar,on Mozilla there is no such a problem. I have couple of synchronous ajax requests and then some info appending on the page,they need about 2 secs to load.During this time the scrollbar freezes and is not usable,when the ajax end the scroll works fine.
If you make multiple AJAX calls, you will receive each response as soon as the server sends it; the order depends on the amount of time that it takes the server to send each reply.
If your code is still running when the server replies, the reply will only be processed after your code finishes.
You should try to load all of the data in a single request.
I might hit a wall here, in other programming languages you can start a new thread for connections/things like that, if you run multiply threads (kinda like layers) your interface stays the way it is/was.
When you use synchronous AJAX the page stops until ajax finish, so if you want the page not stop, must be asynchronous AJAX call.
You can see more here: Documentation AJAX W3Schools