Web Services performance

Web Services performance

I'm developed the Point of Sales (POS) application in Windows and iPad
(iOS devices) app using local database such as MSSQL Server Express and
Sqlite3. These applications work great and FAST in local network
environment. For multi stations I used MSSSQL network server (TCP open
port 1433). For SQLite system, I've wrote a complete WCF server/client
then hosted on local network. Again, the systems work great and FAST. Now
I want explore Web base application which mean that I must hosting web
services on the WWW or Cloud services.
So, my question is what web service protocol should I use to maintaining
the satisfactory POS performance (the applications still have run locally
as native app). I have looks in to JSON, RESTFul protocol but not sure
will give the performance that I need because in POS world it is requires
that fast and a lot database records to SELECT(GET), UPDATE(POST) and
INSERT(PUT) in real-time or close manner.
I know that I can't get a real-time performance on web services but if I
can get 80% plus performance that worth shooting for. Thanks...
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