>We have a client/server application that uses a MySQL database on the server. Some of the clients now connect to the network using an internet dongle, and we saw a real problem with performance on those machines, although the network speed is OK. We figured out that this network has a very slow ping, the traceroute shows it goes first to another island, then to the US, and then back to Aruba and it takes around 700ms to ping.
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>What is the best way to improve performance of a client/server application in this situation? My thought so far was to try to create asynchronous transactions, but this is a lot of work and sometimes not possible.

700ms ping is pretty bad. If you believe in fixing the problem rather than symptoms then that needs to be dealt with. If both clients and the server are on Aruba, find some way to keep the traffic on your island. You could get creative e.g. set up a VPN server which could be accessed from any public WiFi hotspot on the island. Balance the cost of what it might take to re-architect your app vs. putting in a $100 WiFi router.

Another possibility would be to set up a local Terminal Server and have the dongle users connect to that. There would then be no performance issue between the TS and your backend server. But I imagine running RDP or some other remote protocol with 700ms ping times would be pretty brutal.

I'm not sure in the general case you can solve that problem with changes to your app. Yes, async is one way to go but you end up with old-style mainframe batch processing, where you don't know the results until some time later. That may not be practical if you can't proceed with Transaction 2 until you know Transaction 1 completed or rolled back.

Having a crappy Internet connection is their problem, not yours. You're pretty limited in what you can do to alleviate the symptoms. They (or you if you choose to take on that responsibility) need to fix the problem.