I'm getting a bit frustrated about the speed my JSON requests are handled.
I'm getting an average of 1 sec per request to the server. This is too much in my opinion.

Is this because of Ext.Direct ASP.net or is this a common problem with every Ext.Direct?
It's happening because all the calls you were making individually might have been averaging say 200ms and each item would display data as it was loaded. By combining your requests you need to wait until the slowest one is complete before getting anything back.

Try altering the enableBuffer config option in your remoting api. You can set it to send more calls with fewer items in it. This will split up the slower requests (because of a slow stored procedure or whatever).

You could also consider sending the slowest requests individually so that the perceived performance isn't lowered. Because data is loading a user will still think everything is running smoothly...

I'm going to deploy my application tonight on my server and see if the speed changes , maybe it's because I'm working with the development server.
You should see a performance enhancement. I personally always find deployed apps are much quicker - they're not in debug mode and running on an server designed for the job etc...