Does having a master-slave mysql setup over long distances (50-60 ms) slow down the responsiveness of INSERTs or SELECTs on the master locally?
The reads and writes act on the local database. The writes (or any flavor of modification) are written to the binary log, which is read by the slave. So, any activity directly on the master should not be affected by the amount of time it takes for the data to get to the slave.
I currently run a MySQL replication "ring" between central Florida and the Chicago suburbs; each MySQL instance is a master to one instance, and a slave to another, so that all instances are live and contain the data from all the others. The only problem I've seen is that replication lag can cause one of my instances to be "out of date" temporarily because my application is actually faster than the lag.