Data transformation implementation
Is it a cause of degradation of performance? Nobody can really tell until you have measured it. From your description, it is certainly possible, but before I'd get going on the tasks I'd make sure if this is really the case. I'd also weigh in that reimplementing an existing setup doesn't come for free.

As for the steps to do: I'd put in place test cases that make sure that the external applications get the same data when you change something.

Lastly, I'd also get the commitment of management before you start doing that. It wouldn't be the first time that someone tries to put the fault on you if something with the database doesn't, even if you're not involved (and even if it is only marginally related to the database) just for the fact that someone heard that "this guy" is "chaning the system".